High-density FAUs and optical interconnection devices including optimized arrays and related methods

ABSTRACT

A method for fabrication a multifiber cable assembly is provided. The method includes selecting a plurality of optical fibers that each have a respective cladding diameter, determining a maximum fiber core position error for the plurality of optical fibers in a plurality of configurations, and determining a desired order of the plurality of optical fibers that minimizes the maximum fiber position total error.

PRIORITY APPLICATIONS

This application claims the benefit of priority of U.S. ProvisionalApplication No. 63/018,072, filed on Apr. 30, 2020, U.S. ProvisionalApplication No. 63/018,020, filed on Apr. 30, 2020, U.S. ProvisionalApplication No. 63/075,975, filed on Sep. 9, 2020, and U.S. ProvisionalApplication No. 63/143,196, filed on Jan. 29, 2021, the content of whichis relied upon and incorporated herein by reference in entirety.

BACKGROUND Field of the Disclosure

The present disclosure relates generally to optical fibers andconnections. More particularly, this disclosure relates to high-densityfiber array units (FAUs).

Technical Background

Fiber connectors are optical interconnection devices that are used tooptically connect a first optical fiber to a second optical fiber, or afirst set (array) of optical fibers to a second set (array) of opticalfibers. Such fiber connectors are sometimes called fiber-to-fiberconnectors. The optical fibers are typically carried by optical fibercables (“cables”). Cables that carry multiple optical fibers are calledmultifiber cables. Cables where the optical fibers are carried in rowsand which are relatively flat are called fiber ribbon cables or just“ribbon cables.”

Fiber connectors may also be used to optically connect an array ofoptical fibers carried by a multifiber cable to an array of opticalwaveguides of a planar light circuit (PLC) or an integrated photonicdevice such as a photonic integrated circuit (PIC). Such fiberconnectors are sometimes called fiber-to-chip connectors.

Because optical fibers have relatively small core diameters, e.g., onthe order of 10 microns for single mode optical fibers, fiber-to-fiberconnectors and fiber-to-chip connectors need to establish alignment withtheir optical counterpart connectors to submicron accuracy. Fiberconnectors configured to connect multiple optical fibers such as carriedby a multifiber cable are referred to as multifiber connectors.

A conventional approach to achieving precision alignment of an array ofoptical fibers in a multifiber connector is to use a V-groove substratemachined from flat glass. Unfortunately, fabricating V-groove substratesis expensive and time consuming and requires the use of expensivemachine tools. As it is anticipated that multifiber connectors will findincreasing use for a variety of applications that would benefit fromleveraging the data-carrying capacity of multifiber cables, there is aneed for low-cost manufacturing solutions for forming multifiberconnectors that may still provide the required alignment precision whenmaking optical interconnections between arrays of optical fibers orbetween an array of optical fibers and an array of waveguides of a PLCor PIC.

SUMMARY OF THE DETAILED DESCRIPTION

The present disclosure discloses compact, solder reflow compatible, FAUsbased on lidless fiber array squeeze technology, and processes tofabricate the same. These lidless FAUs may be well suited for passivealignment to PICs. In some high bandwidth data center switches,co-location of many compact optoelectronic transceivers aroundelectronic switch chips on a common interposer substrate or multi-chipmodule is becoming a common practice. Lidless FAUs enable a variety ofoptical interconnections to PICs used in optoelectronic transceivers,including edge, evanescent, and grating coupling solutions. Eliminationof the expensive glass V-groove substrate normally found in an FAU helpsreduce material cost, while the squeeze approach enables arrangement ofoptical fibers on a fine pitch for compact high density PICinterconnections.

In an example embodiment, a FAU may be fabricated with an integralsupport sheet that stiffens the fiber array. An interdigitated barefiber array may be arranged on top of a rigid support sheet and anadhesive applied. A release pad may be disposed on the exposed surfaceof the bare fibers. The FAU may then be placed between a top plate and abottom plate. Force is applied on all four sides of the fiber array toforce adjacent fibers into contact with each other. The vertical squeezeforce, applied to the top plate and bottom plate, ensures that allfibers in the fiber array are in contact with the top surface of thesupport sheet. A horizontal squeeze force may be applied to the barefibers by pusher sheets ensuring contact between adjacent opticalfibers. The adhesive may be a UV-curable adhesive which is cured byapplying ultraviolet (UV) light, heat, or the like. After the adhesivehas cured the force and release pad may be removed, resulting in alidless FAU that includes datum contacts between each of the adjacentoptical fibers of the fiber array and between each of the optical fibersand the support substrate. Further, the fiber array includes an exposeddatum surface disposed at a top surface of each optical fiber of thefiber array and/or an exposed datum surface on each edge optical fiber.These exposed datum surfaces may be mated with precision surfaces, suchas on a waveguide substrate to enable highly accurate passive alignmentof the optical fibers of the FAU to waveguides of the waveguidesubstrate.

In some example embodiments, the optical fibers of the fiber array maybe organized according to the respective outside diameters of the fibercladding prior to ribbonization. The fiber array may be compared to anideal core position in a first configuration. to determine a coreposition error The positions of two fibbers of the fiber array may beswapped and compared to the ideal core position to determine a secondcore position error. The process may be iterated a predetermined numberof time to determine a fiber array configuration having the smallest,e.g. optimized, core position error. By optimizing the order of theoptical fibers that comprise the ribbon, the core-to-core pitch errormay be minimized resulting in significantly less core-to-core pitcherrors in the FAU. This may be especially true in a large array FAU,such as 24-96 fibers. In an example embodiment, a group of opticalfibers may be selected and a simulation of core-to-core pitch errorsbased on the cladding measurements for each of the optical fibers may becarried out. The optical fibers may then be arranged in an order thatminimized core-to-core pitch error prior to ribbonization.

Lidless or lid-optional FAUs of the present disclosure compared toV-grooves are relatively more compact, easier to prepare in advance, andmore practical for use in pick-and-place applications.

In still further example embodiments, a fiber optic assembly is providedincluding a support substrate having a first surface comprising aplurality of V-grooves and a signal-fiber array supported on the firstsurface of the support substrate. The signal-fiber array including aplurality of optical fibers disposed in the plurality of V-grooves. Thefiber optic assembly also including an adhesive disposed on theplurality of optical fibers and the support substrate. A first datumsurface is disposed at a top surface of each of the plurality of opticalfibers opposite the support surface.

In another example embodiment, a fiber optic assembly is providedincluding a support substrate having a substantially planar surface anda signal-fiber array supported on the planar surface of the supportsubstrate. The signal-fiber array includes a plurality of opticalfibers. The fiber optic assembly also includes an adhesive disposed onthe plurality of optical fibers and the support substrate. Each of theoptical fibers is spaced from adjacent optical fibers of the pluralityof optical fibers at a precise pitch.

In a further embodiment, a fiber optic assembly is provided including asupport substrate having a planar surface and a signal-fiber arraysupported on the planer surface of the support substrate. Thesignal-fiber array includes a plurality of optical fibers. The fiberoptic assembly also includes an adhesive disposed on the plurality ofoptical fibers and the support substrate. Each of the optical fibers isspaced from adjacent optical fibers of the plurality of optical fibersand a datum surface is disposed on an outer surface of each edge opticalfiber of the plurality of optical fibers.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be apparent to those skilledin the art from the description or recognized by practicing theembodiments as described in the written description and claims hereof,as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understand the natureand character of the claims.

The accompanying drawings are included to provide a furtherunderstanding, and are incorporated in and constitute a part of thisspecification. The drawings illustrate one or more embodiment(s), andtogether with the description explain the principles and operation ofthe various embodiments.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a perspective view of an FAU in an initial stage ofassembly according to an example embodiment.

FIG. 2 illustrates a cross-sectional view of an example lidless FAUafter an assembly process according to an example embodiment.

FIGS. 3-7B illustrate cross-sectional views of the lidless FAU of FIG. 2during each phase of the assembly process using a U-shaped release padaccording to an example embodiment.

FIG. 8 illustrates a cross-sectional view of an assembly process of alidless FAU using a planar release pad according to an exampleembodiment.

FIGS. 9A and 9B illustrate cross-sectional views of assembly of alidless FAU including D-shaped optical fibers according to an exampleembodiment.

FIGS. 10A and 10B illustrate cross-sectional views of assembly of alidless FAU including D-shaped optical fibers according to an exampleembodiment.

FIG. 11 illustrates a perspective view of a U-shaped release pad andsheet pushers according to an example embodiment.

FIGS. 12 and 13 illustrate perspective views of an assembly of a lidlessFAU according to an example embodiment.

FIGS. 14A and 14B illustrate a top down and side view, respectively of alidless FAU including an adhesive strain relief according to an exampleembodiment.

FIG. 15 illustrates passive alignment of a lidless FAU to a waveguidesubstrate having a notch feature according to an example embodiment.

FIG. 16 illustrates passive alignment of a lidless FAU to a notchfeature having precision surfaces according to an example embodiment.

FIGS. 17A and 17B illustrate passive alignment of a lidless FAU havingD-shaped optical fibers to a notch feature according to an exampleembodiment.

FIGS. 18A and 18B illustrate example passive alignments of a lidless FAUto a notch feature including spacer fibers according to an exampleembodiment.

FIG. 19 illustrates passive alignment of a lidless FAU to a notchfeature having an underetch according to an example embodiment.

FIG. 20 illustrates passive alignment of a lidless FAU to a notchfeature having a precision surface on sidewalls and a bottom surfaceaccording to an example embodiment.

FIG. 21 illustrates passive alignment of a lidless FAU to a waveguidesubstrate including an alignment feature according to an exampleembodiment.

FIG. 22 illustrates that FAU fabrication process including a steppedpusher sheet according to an example embodiment.

FIG. 23 illustrates a lidless FAU installation onto a flip chip mountedPIC substrate according to an example embodiment.

FIG. 24 illustrates the lidless FAU of FIG. 23 installed onto the flipchip mounted PIC substrate according to an example embodiment.

FIG. 25A illustrates fabrication of an alignment substrate according toan example embodiment.

FIG. 25B illustrates an example alignment substrate according to anexample embodiment.

FIG. 26A-26C illustrate bottom, side, and end views of an exampleinstallation of an alignment substrate onto a PIC face of a PICsubstrate according to an example embodiment.

FIG. 27A-27C illustrate top, side, and end views of the alignmentsubstrate installed on the PIC substrate according to an exampleembodiment.

FIGS. 28A and 28B illustrate installation of an overlap sheet on alidless FAU according to an example embodiment.

FIGS. 29A-29C illustrate top, side, and end views of installation of alidless FAU including an overlap sheet to a PIC substrate according toan example embodiment.

FIGS. 30A-30C illustrates top, side, and end views of the lidless FAU ofFIGS. 29A-29C installed on the PIC substrate according to an exampleembodiment.

FIG. 31 illustrates and example lidless FAU installed onto a PICsubstrate that includes a cap according to an example embodiment.

FIGS. 32A-32C illustrate top, side, and end views of installation of alidless FAU including an to a PIC substrate that includes a retentionclip according to an example embodiment.

FIGS. 33A-33C illustrates top, side, and end views of the lidless FAUinstalled on the PIC substrate of FIGS. 32A-32C according to an exampleembodiment.

FIGS. 34A-34C illustrates top, side, and end views of a gripper toolengaging the lidless FAU installed on the PIC substrate of FIGS. 32A-32Caccording to an example embodiment.

FIGS. 35A and 35B illustrate a comparison of an ideal optical fiber andan optical fiber with cladding diameter and core eccentricity variationaccording to an example embodiment.

FIGS. 36A and 36B illustrate a comparison of an FAU fabricated withideal optical fibers and an FAU fabricated with optical fibers havingcladding diameter and core eccentricity variation according to anexample embodiment.

FIG. 37 illustrates a determination of fiber core position erroraccording to an example embodiment.

FIG. 38 illustrates an example FAU including eight randomly selectedoptical fibers according to an example embodiment.

FIG. 39 illustrates a data plot of one thousand random FAUconfigurations for different signal-fiber array sizes according to anexample embodiment.

FIG. 40 illustrates a data plot of maximum fiber core total error versusthe size of the signal-fiber array according to an example embodiment.

FIG. 41 illustrates iterations of an algorithm that randomly swaps twooptical fibers in a signal-fiber array to determine an optical fiberorder that minimizes the maximum fiber core total error according to anexample embodiment.

FIG. 42 depicts iterations of an algorithm that randomly swaps twooptical fibers in a signal-fiber array and a pool of optical fibers todetermine an optical fiber order that minimizes the maximum fiber coretotal error according to an example embodiment.

FIG. 43 illustrates a data plot for a signal-fiber array includingthirty-two optical fiber fabricated from two multifiber cables eachincluding sixteen optical fibers according to an example embodiment.

FIG. 44 illustrates a fabrication process for a signal-fiber arrayincluding thirty-two optical fiber formed from two multifiber cableseach including sixteen optical fibers according to an exampleembodiment.

FIG. 45 illustrates FAU including a signal-fiber array formed from aplurality of fiber array groups with spacer fibers between the fiberarray groups according to an example embodiment.

FIG. 46 illustrates an apparatus for determining a maximum fiber coretotal error for a plurality of optical fibers in a plurality ofconfigurations according to an example embodiment.

FIGS. 47 and 48 depict an example process for forming a lidless FAUusing a V-groove support substrate according to an example embodiment.

FIGS. 49-51 depict cross-sectional views of the process of FIGS. 47 and48 according to an example embodiment.

FIGS. 52A-53B illustrate various adhesive profiles of a lidless FAU on aV-groove support substrate according to an example embodiment.

FIGS. 54 and 55 depict an example process for forming a lidless FAUusing a reusable V-groove alignment substrate according to an exampleembodiment.

FIGS. 56 and 57 depict cross-sectional views of the process of FIGS. 54and 55 according to an example embodiment.

FIG. 58 depicts an FAU including an optional lid disposed on an adhesiveprofile according to an example embodiment.

FIG. 59 illustrates a sanded V-groove adhesive profile according to anexample embodiment.

FIG. 60 illustrates the FAU of FIG. 59 including an optional lidaccording to an example embodiment.

FIGS. 61-63 depict cross-sectional views of forming a lidless FAU usinga reusable truncated V-groove alignment substrate according to anexample embodiment.

FIG. 64 depicts the FAU of FIGS. 61-63 including an optional lidaccording to an example embodiment.

FIG. 65 depicts the FAU of FIGS. 61-63 including an optional lid withtrench according to an example embodiment.

FIGS. 66A and 66B illustrate passive alignment of an FAU with truncatedV-groove adhesive profiles to a PIC substrate according to an exampleembodiment.

FIGS. 67 and 68 illustrate a process of forming a lidless FAU with datumsurfaces on the top and outboard optical fibers according to an exampleembodiment.

FIG. 69 illustrates passive alignment of the FAU of FIGS. 67 and 68 witha PIC substrate according to an example embodiment.

FIGS. 70 and 71 depict a cross-sectional view of the passive alignmentof FIG. 69 according to an example embodiment.

FIGS. 72 and 73 depict a cross-sectional view of passive alignment of alidless FAU with vertical offset space formed in the adhesive profileaccording to an example embodiment.

FIGS. 74-76 illustrate a process for forming a lidless FAU includingalignment features formed in the adhesive profile according to anexample embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to certain embodiments, examples ofwhich are illustrated in the accompanying drawings, in which some, butnot all features are shown. Indeed, embodiments disclosed herein may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Whenever possible, like reference numbers will be used torefer to like components or parts.

The claims set forth below are incorporated by reference into thisDetailed Description section.

Terms like front, back, top, bottom, side, etc. are relative terms usedfor convenience and ease of explanation and are not intended to belimiting.

A brief explanation of selected terminology used herein is nowpresented.

The abbreviation μm stands for “micron” or “micrometer,” while theabbreviation nm stands for nanometer.

As used herein, the term “datum surface” means a fixed reference pointor surface free of debris or other foreign materials, such as to enablea direct contact between a first structure and a second structure.Additionally, as used herein, the term “datum contact” shall mean thedirect contact of a datum surface with another structure.

As used herein, the term “precision surface” means a surface that issubstantially planner having deviations from a flat plane of less than0.5 μm over the surface.

As used herein, the term “precise pitch” of the fiber cores means apitch sufficient to guarantee low loss coupling between FAU waveguidesand waveguides of another component. For example, for single mode fibercoupling a precise pitch would be <1 μm deviation in X and Y from anideal grid (laid out on 127 μm or 250 μm pitch). Preferably thedeviation from the ideal grid would be <0.7 μm or <0.5 μm.

Cartesian coordinates are shown in some of the Figures for the sake ofreference and are not intended to be limiting as to direction ororientation. Likewise, use of relative terms such as “top,” and “bottom”and “side” and “edge” and the like are used herein for ease ofdescription and explanation and are not intended to be limiting as to adirection or orientation.

As used herein, the term “interdigitated”, with respect to opticalfibers, means that a first group of optical fibers and a second group ofoptical fibers (e.g., groups A and B) are arranged in an alternatingpattern (e.g., A-B-A-B-A-B . . . ).

References to “fiber core position” or “core position” refer to theposition of the center of a core of an optical fiber.

The present disclosure concerns fiber array units for opticalinterconnections. These fiber array units may include a plurality ofoptical fibers disposed on a support substrate. The plurality of opticalfibers may include datum contacts between adjacent optical fibers andbetween each of the optical fibers and the support substrate.Additionally, each of the optical fibers may include an exposed datumsurface on a top surface opposite the support substrate.

Lidless Fiber Array Unit (FAU)

FIG. 1 is a perspective view of an example fiber optic assembly, orfiber array unit (FAU) 10, at an initial stage of formation. The FAU 10includes a planar support substrate 20 having a top surface 22, a bottomsurface 24, a front end 26, a back end 28 and opposite edges 30. Thesupport substrate has dimensions width (W), length (L), and height (H).The front end 26, the back end 28 and the opposite edges 30 define aperimeter 32 of the support substrate 20. Lidless means a lid is notnecessary, but is optional for the FAUs. A clear plastic lid or othersuitable material, may be used for observation of the optical fibers oras a temporary cover for protective purposes.

The support substrate 20 may be fabricated from a material that providesflat surface, such as polished glass, ceramic, or metal materials. In anexample embodiment, glass is a preferred material for the supportsubstrate 20 because glass may be formulated with a coefficient ofthermal expansion (CTE) that is a close match to that of siliconmaterials commonly used in active photonic components. While othermaterials, such as silicon, may also be used, glass may be particularlyadvantageous because glass is transparent to visible light, simplifyingalignment of optical fibers 52 on the support substrate 20 and allowingtransmission of UV light, enabling UV curing of a UV-curable adhesive(discussed below in reference to FIGS. 2-7B). In some exampleembodiments, a glass support substrate 20 may be fabricated using aprocess that ensures a high degree of flatness with minimal surfaceroughness, e.g., Ra<0.1 micron, where Ra is the arithmetic average ofthe absolute values of the profile height deviations from a mean line,recorded within the evaluation length. Said differently, Ra is theaverage of a set of individual measurements of the peaks and valleys ina surface taken over a given length. One suitable method for fabricatingsupport substrate 20 is traditional glass grinding and polishingprocesses. Another suitable fabrication method uses a fusion drawprocess used for liquid crystal display (LCD) glass fabrication. Thefusion draw process may also produce glass sheets with parallel top andbottom surfaces and precise thickness control. A further suitablefabrication method may be float glass forming. Support substrate 20 maybe monolithic or made in sections, and the outer surface may havesurface features such as smoothness, porosity, roughness, striations,recesses, or grooves, and combinations thereof, to enhance bonding orfiber alignment.

The support substrate 20 may be small and thin (e.g., W=3 mm, L=10 mmand H=1 mm) in some embodiments to facilitate fabrication of compactfiber array assemblies and interconnect devices such as connectors. Inan example, the height (H) of the support substrate 20 may be the rangefrom 0.7 mm to 1.0 mm, but this height (H) may be smaller or greaterdepending on the particular application requirements. For example, itmay advantageous for the support substrate 20 to have a height (H) thatprovides for sufficiently rigid support for the fiber arrays introducedand discussed herein below. In some cases, glass support substrates 20with a height (H) less than about 0.4 mm may tend to deflect duringassembly and use, resulting in unacceptable out-of-plane fiberalignment, known as “potato-chipping.” Conversely, since sheet stiffnessincreases with the cube of the sheet thickness, relatively large fiberarrays (in terms of number of fibers) may be accommodated through onlymodest increases in the height (H) of the support substrate 20.

The support substrate 20 may be cut to size from a larger sheet, forexample, using a computer-controlled dicing saw with a diamond blade orby laser cutting. A single sawing or cutting operation carried out on awafer or glass sheet sample may yield hundreds of support substrates 20.After sawing, the support substrates 20 may be bevel edge ground orlightly sanded around their edges to round off sharp corners that mightotherwise damage bare optical fibers during the assembly process. Thesupport substrates 20 may then be cleaned manually by wiping thesurfaces with an ethanol-soaked wipe. The support substrates 20 may alsobe cleaned in an oxygen plasma furnace to completely remove all organicmaterials from the surfaces and to prepare the top surface forsubsequent bonding using an adhesive, such as an organic adhesive.

With continuing reference to FIG. 1 , the FAU 10 also includes asignal-fiber array 50 having signal optical fibers, or “optical fibers”52. The optical fibers 52 have respective front-end sections 54 thatinclude respective front ends 55 that have respective end faces 56. Thesignal-fiber array is referred to as a “signal-fiber arrays” because thecorresponding optical fibers 52 are configured to carry one or moreoptical signals.

As shown in detail A, each optical fiber 52 comprises a core 72, acladding 74 surrounding the core 72, and a protective coating 76surrounding the cladding 74. In an example, the protective coating 76 isstripped way to define “bare glass” optical fiber sections (where theformer protective coating 76 outer surface is indicated by the dashedlines). Thus, in an example, the front-end sections 54 of the opticalfibers 52 are formed as bare-glass front-end sections.

In an example, the signal-fiber array 50 is respectively supported by amultifiber cable 60 that has a cable jacket 61. A matrix material (notshown) may be applied to the signal fiber array 50 that encapsulatesoptical fibers 52 or may be applied intermittently, e.g. spider webribbons. In some example embodiments, the In the example of FIG. 1 ,front-end portions of the cable jacket 61 is stripped away to access theoptical fibers 52. In addition, the protective coatings 76 are alsoremoved (stripped) from the front-end sections 54 of the optical fibers52 to form the bare-glass front-end sections 54. The stripping processesfor removing the cable jacket 61 may be carried out using mechanicalstrippers, which heat and soften the cable jacket prior to removal usinga pair of serrated blades. The matrix material and/or the protectivecoating 76 may be removed using a similar mechanical process or alaser-based stripping process. In an example, the multifiber cable 60may comprise a ribbon cable.

After the cable jackets 61 and the protective coating are stripped away,the exposed portions of the signal-fiber array 50 may be cleaned. In anexample, this is accomplished by sandwiching the optical fibers 52between a folded lint-free wipe that has been soaked in ethanol, andthen drawing the wipe toward the front ends 55 of the optical fibers.The signal-fiber array 50 may also be cleaned using an oxygen plasma.

When the front-end portions of the cable jacket 61 are removed from themultifiber cable 60, the optical fibers 52 tend to curl in onedirection, as illustrated in FIG. 1 for the multifiber cable 60 and thesignal-fiber array 50. This curl arises from fiber ribbon manufacture.The ribbonized multifiber cable 60 may be wound around a core or drum,and, therefore, curl in the same direction.

Therefore, it may be advantageous, when forming the FAU 10, that thesignal-fiber array 50 is arranged such that the curl of the opticalfibers 52 is directed downward, i.e., towards the top surface 22 of thesupport substrate 20, as shown in FIG. 1 . This ensures that when thesignal-fiber array 50 is brought into proximity to the support substrate20, the front ends 55 of the optical fibers 52 contact the top surface22 of the support substrate 20.

FIG. 2 illustrates a cross-sectional view of an assembled FAU 10according to an example embodiment. The FAU 10 may include asignal-fiber array 50 including a plurality of optical fibers 52. Theplurality of optical fibers 52 may be supported by a support substrate20. An adhesive 70 may be provided in voids between adjacent opticalfibers 52 and between the optical fibers 52 and the support substrate20. The adhesive 70 may be a UV-curable adhesive or other suitableadhesive. The adhesive 70 may bond the optical fibers 52 to each otherand/or to the support substrate 20. In some example embodiments, otherbonding technologies may be employed, including but not limited to laserbonding, liquid glass (sodium silicate) bonding, or anodic bonding (forglass fiber to silicon substrate).

In an example embodiment, the signal-fiber array 50 may be formed fromone or more multifiber cables 60. In embodiments in which multiplemultifiber cables 60 are employed, the optical fibers 52 of each of themultifiber cables 60 may be interdigitated. The bare-glass front-endsections 54 of the optical fibers 52 may be in direct contact with thesupport substrate 20 defining a first datum contact 101 between each ofthe plurality of optical fibers 52 and the support substrate 20.Additionally, each of the optical fibers 52 may be in direct contactwith at least one adjacent optical fiber 52 defining a second datumcontact 102 therebetween.

In the embodiment shown, the FAU 10 does not require a lid, such as aglass lid or other substrate, disposed on the top of the optical fibers52 opposite the support substrate 20. This enables each of the pluralityof optical fibers 52 to define a first exposed datum surface 103 at atop of the optical fibers 52 opposite the support substrate 20. Theabsence of the lid, the FAU 10 may have a shallower profile and therebyenable an increased density of optical connections. Additionally, thedatum contacts 102 between each of the plurality of optical fibers 52may enable highly accurate alignment of the FAU 10 with a waveguidesubstrate or other optical components, such as mating with a precisionsurface disposed on the waveguide substrate.

In some example embodiments, the FAU 10 may include a second exposeddatum surface 104 disposed on each edge optical fiber 52, e.g. opticalfibers 52 that have only one adjacent optical fiber 52. The secondexposed datum surface 104 may enable lateral alignment of the FAU 10with the waveguide substrate, such as mating with a precision surfacedisposed on the waveguide substrate.

The adhesive 70 bonding the optical fibers 52 to the support substrate20 and to adjacent optical fibers 52 may reside below a plane defined bythe first exposed datum surface 103 and/or the second exposed datumsurfaces 104, such as to not interfere with any mating of the firstexposed datum surface 103 or the second exposed datum surface 104. Theadhesive hardness disclosed herein, for example, has a Shore D of about65-80 as determined by ASTM D2240-00 and a bonding strength of about2,000-3600 psi determined by ASTM D3165.

FIGS. 3-7B illustrate an example method of fabricating the FAU 10depicted in FIG. 1 . As depicted in FIG. 3 , bare-glass front-endsections 54 of the optical fibers 52 may be placed in direct contactwith the support substrate 20. The support substrate 20 may bepositioned on bottom plate 200, such as a glass bottom plate or othersuitable material. It is advantageous for the bottom plate 200 to betransparent to light, such that transmission of UV light may enable UVcuring of a UV-curable adhesive 70. A UV-curable adhesive 70 may beselected that is stable through a solder reflow process associated withthe Photonic Integrated Chip (PIC). The adhesive 70 may therebystabilize a mechanical interface between adjacent optical fibers 52 ofthe FAU 10, between the optical fibers 52 and the support substrate 20,and in subsequent assembly steps, between the FAU 10 and a PIC throughsolder reflow conditions. Additionally or alternatively, the FAU 10 maybe passively aligned with the PIC substrate after a solder reflowprocess. Although, UV adhesive is discussed herein, other adhesives maybe used, such as thermal set, epoxy, or the like.

The adhesive 70 may be applied to the bare-glass front-end sections 54of the optical fibers 52 and/or the support substrate, such that theadhesive flows around to contact all sides of each optical fiber 52 inthe signal-fiber array 50. During application of the adhesive 70, a topplate 202 and an associated elastic release pad 204 may be in aretracted position. Similarly, pusher sheets 206 may also be retracted.The top plate 202 may be formed of metal, glass, or other suitablematerial to transfer pressure from an actuator to the release pad 204,without deformation, to provide an even distribution of pressure acrossthe release pad 204. The release pad 204 may be formed of a deformablenon-stick material, such as silicone. The thickness of the release pad204 may be selected to allow for deformation of the release pad 204under application of force applied by the top plate 202 during an FAUassembly process. The deformation of the release pad 204 helps controland limit the flow of the adhesive 70 around the squeezed signal-fiberarray 50 during assembly. Adhesive 70 may be excluded from regionsbetween and on top of individual optical fibers 52 of the signal-fiberarray 50 to allow the exterior exposed surfaces of the optical fibers 52to serve as datum surfaces, as discussed above. The release pad 204 maynot need to have a precise thickness because the release pad 204 iscompressed during the assembly process. The release pad 204 may also becovered with a conformal non-stick release sheet, such aspolytetrafluoroethylene (PTFE), or a non-stick coating, such as afluorosilane coating.

In some example embodiments, the release pad 204 may be substantiallyplanar, configured to exclude adhesive from the top surfaces of thesignal-fiber array 50. In another example embodiment, the release pad204 may have a generally U-shape, having a center channel, or cavity,that is roughly the width of the target signal-fiber array 50 andportions that extend downward on either side of the signal-fiber array50, such as depicted in FIG. 3 . In an example embodiment, the internalcavity width of the U-shaped release pad 204 may be selected to beslightly wider than the signal-fiber array 50, such as 25-50 μm wider,so that it may slip over the signal-fiber array 50 during assemblywithout interference. The side portions of the release pad 204 may becompressed by the pusher sheets 206 during the FAU assembly process. Theside portions of the release pad 204 may deform around the edge opticalfibers 52 to exclude or substantially avoid adhesive on the exposed edgesurfaces of the edge optical fibers 52. The exposed exterior edgesurfaces of the optical fibers 52 may serve as datum surfaces aftercompletion of the assembly process.

The release pad 204, may be held in contact with the top plate 202using, for example, vacuum forces, a gripping fixture that holds therelease pad, or a temporary adhesive. The release pad 204 may be atleast 10-20 μm thick and preferably 30-50 μm thick in someconfigurations. In other configurations, the release pad 204 may beformed thicker, such as 100-500 μm thick.

The pusher sheet 206 may a substantially planar material formed fromglass, metal, plastic, or the like. The pusher sheet 206 may have athickness of about the diameter of the optical fibers. In an exampleembodiment, the pusher sheet 206 may have a thickness up to theuncompressed thickness of the release pad 204. In some exampleembodiments, a fiber array similar to the signal-fiber array 50 may beused as the pusher sheet 206. The pusher sheet 206 may have a rigidityand strength sufficient to allow for numerous repetitions of the FAUassembly process with little or no damage or deformation.

Turning to FIG. 4 , the top plate 202 and associated release pad 204 maybe moved down toward the bottom plate 200. The release pad 204 may be incontact with the signal-fiber array 50 on a top surface and/or incontact with the support substrate 20 on either side of the signal-fiberarray 50. Excess adhesive 70 may be displaced away from the signal-fiberarray 50 and/or the surface of the support substrate 20 by the releasepad 204.

FIG. 5 depicts force being applied on all four sides of the signal-fiberarray 50 to force adjacent optical fibers 52 into contact with eachother, thereby creating the first datum contacts 101. The verticalsqueeze force may ensure that each of the optical fibers 52 fibers inthe signal-fiber array 50 are in contact with the top surface of thesupport substrate 20, thus generating the second datum contacts 102. Asthe signal-fiber array 50 is pushed into the elastic release pad 204,adhesive 70 is excluded from regions between the optical fibers 52 onthe top half of the signal-fiber array 50. As the pusher sheets 206 pushon the side portions of the U-shaped release pad 204, the side portionsof the U-shaped release pad 204 come into contact with the two outermostoptical fibers 52, e.g. edge optical fibers of the signal-fiber array50, deforming around the edge optical fibers 52 and excluding orsubstantially avoiding adhesive 70 on the outside edges of the edgeoptical fibers 52.

In FIG. 6 , a UV light, or beam, is transmitted through the bottom plate200 and support substrate 20. The UV light causes the UV curing of theUV-curable adhesive 70. The adhesive 70 may be UV cured whilemaintaining the top, bottom, left, and right squeeze forces. After theadhesive 70 cures, the pusher sheets 206 and the U-shaped release pad204 may be removed, as depicted in FIG. 7A. The FAU 10 may then beremoved from the glass bottom plate 200, as shown in FIG. 7B. Using theU-shaped release pad 204 causes the top surface of the plurality ofoptical fibers 52 to include the first exposed datum surface(s) 103.Further, use of the side portions of the U-shaped release pad 204 maycause the outer edges of the edge optical fibers 52 to include thesecond exposed datum surface(s) 104. The exposed datum surfaces 103, 104of the FAU 10 may be free of adhesive, allowing these regions to be usedas precision alignment datum surfaces. In some embodiments, deformationof the release pad 204 about a portion of the optical fibers during theassembly process causes the adhesive to be cured below a plane definedby the exposed datum surfaces 103, 104. In an example embodiment, theside portions of the release pad 204 may also exclude the adhesive fromthe support substrate 20 in an area adjacent to the edge optical fibers,resulting in an adhesive free surface 208. Subsequent dicing andpolishing operations may be carried out, in some embodiments, to preparethe end faces 56 of the optical fibers 52 for low loss optical coupling.It is also contemplated that the assembly process may also be carriedout in a configuration that is inverted from the approach shown in FIGS.3-7B, where the release pad 204 is on the bottom, the optical fibers 52are in the middle, and the support substrate 20 is on top.

As discussed above, in some example embodiments, the release pad 204 maybe generally planar. FIG. 8 depicts an FAU 10 in which the signal-fiberarray 50 is sandwiched between the top plate 202 and the bottom plate200, using a planar release pad 204A. The release pad, or in this casethe planar release pad 204A, may be held in contact with the top plate202 using, for example, vacuum forces, a gripping fixture that holds therelease pad, or a temporary adhesive. The planar release pad 204A may beat least 10-20 μm thick and preferably 30-50 μm thick in someconfigurations. In other configurations, the planar release pad 204A maybe formed thicker, such as 100-500 μm thick.

In this embodiment, the pusher sheets 206 apply force directly to theedge optical fibers 52 of the signal-fiber array 50. As such, the pushersheets 206 may be fabricated from a non-stick material (e.g., PTFE). Inan example embodiment, the pusher sheets 206 may be formed with anelastic material at the tips of each of the pusher sheets 206 to enhanceadhesive removal.

Similar to the embodiment utilizing a U-shaped release pad 204,discussed above and depicted in FIG. 7A, using the planar release pad204A may cause the top surface of the plurality of optical fibers 52 toinclude the first exposed datum surface(s) 103. Further, the pushersheets 206 may cause the outer edges of the edge fibers to include thesecond exposed datum surface(s) 104. The exposed datum surfaces 103, 104of the FAU 10 may be free of adhesive, allowing these regions to be usedas datum surfaces for subsequent passive or active alignment to PICs.

Turning to FIGS. 9A and 9B, the lidless FAU assembly process may beutilized with D-shaped optical fibers 52A. In this configuration, theD-shaped optical fibers 52A are arranged with a flat portion orientedupward, toward the top plate 202 and a round portion oriented toward thesupport substrate 20. A thin planar release pad 204A may be used topress the D-shaped optical fiber into contact with the supportsubstrate, such as to generate the second datum contacts 102. In anexample embodiment, the surface of the support substrate 20 may be aprecision surface. Pusher sheets 206 may provide lateral force tosqueeze the D-shaped optical fibers 52A together from the sides, so thatthe D-shaped optical fibers 52A come into contact with each other togenerate the first datum contacts 101. Adhesive 70 may fill the voidsbetween the D-shaped optical fibers 52A and the support substrate 20.After UV-curing of the adhesive 70, the pusher sheets 206 and planarrelease pad 204A may be removed (shown in FIG. 9B). The FAU 10 includingD-shaped optical fibers 52A may include the first exposed datum surfaces103 on the top surface, being the flat portions of the D-shaped opticalfibers 52A, and the second exposed datum surfaces at the edge D-shapedoptical fibers 52A. for subsequent passive or active alignment to PICs.

Using a similar assembly approach, D-shaped optical fibers 52A may bepositioned on the support substrate 20 with the flat portions orienteddownward toward the support substrate 20, as shown in FIGS. 10A and 10B.A thin planar release pad 204A may be used to press the D-shaped opticalfibers 52A into contact with the support substrate 20 to generate thesecond datum contacts 102. In an example embodiment, the surface of thesupport substrate 20 may be a precision surface. Pusher sheets 206 mayprovide lateral force to squeeze the D-shaped optical fibers 52Atogether from the sides, so that the D-shaped optical fibers 52A comeinto contact with each other to generate the first datum contacts 101.Adhesive 70 may fill the voids between the D-shaped optical fibers 52Aand the support substrate 20. After UV-curing of the adhesive 70, thepusher sheets 206 and planar release pad 204A may be removed (shown inFIG. 10B). The FAU 10 including D-shaped optical fibers 52A may includethe first exposed datum surfaces 103 on the top surface, being the flatportions of the D-shaped optical fibers 52A, and the second exposeddatum surfaces at the edge D-shaped optical fibers 52A for subsequentpassive or active alignment to PICs.

FIG. 11 illustrates a perspective view of an example U-shaped releasepad 204 according to an example embodiment. The release pad 204 mayinclude a straight channel region (SR) formed to be slightly larger thanthe FAU 10, such that the optical fibers 52 may be inserted withoutsignificant interference. In an example embodiment, the straight channelregion SR may be approximately 1-2 mm. The release pad 204 may alsoinclude a tapered channel region (TR), such as a 20 degree taper. Thetapered channel region TR may allow for a gradual change in spacing ofthe optical fibers 52 from the multifiber cable 60, which includes theprotective coating 76 to the FAU 10, where the optical fibers 52 are indirect contact with each other and the protective coating 76 is removed.The pusher sheets 206 may be positioned such that when actuated, thepusher sheets 206 push on the straight channel region SR of the releasepad 204. A vacuum tip 210 (FIG. 12 ) may be utilized to hold the supportsubstrate 20 over the release pad 204. Similar to the pusher sheets 206,the vacuum tip 210 is positioned over the straight channel region SR ofthe release pad 204 to allow the force to be concentrated over thestraight channel region SR where the optical fibers are squeezedtogether during the FAU assembly.

Turning to FIG. 12 , the support substrate 20 is being held over therelease pad 204 by the vacuum tip 210. A signal-fiber array 50 of aribbon cable, e.g. multifiber cable 60, is stripped of the protectivecoating 76 to expose the bare-glass front-end sections 54, andUV-curable adhesive 70, such as Epotek 353HD, is applied over thebare-glass front-end sections 54. Adhesive 70 may be applied bypositioning the signal-fiber array 50 between lint-free pads that havebeen soaked with adhesive 70. The adhesive soaked pads may be lightlycompressed over the top and bottom of the bare-glass front-end section54, so that the adhesive 70 is transferred to the bare-glass front-endsections 54. As the signal-fiber array 50 is removed from the adhesivesoaked pads, a thin layer of adhesive 70 is applied over the bare-glassfront-end sections 54 of the signal-fiber array 50.

After application of the adhesive 70 on the bare-glass front-endsections 54 of the signal-fiber array 50, the signal-fiber array 50 islowered, such that it contacts the release pad 204 in the taperedchannel region TR. The support substrate 20 may then be lowered onto therelease pad 204, such that the signal-fiber array is constrained in thetapered channel region TR of the release pad 204 by the supportsubstrate 20, as shown in FIG. 13 (vacuum tip 210 is not shown forclarity of the depicted details). The signal-fiber array 50 is thenslowly moved down the tapered channel region TR to the straight channelregion SR (located under the support substrate 20 and vacuum tip 210),such that the ends of the optical fibers 52 become squeezed together.

The signal-fiber array 50 may be pushed further down the channel of therelease pad 204 until the signal-fiber array 50 emerges from the far endof the channel of release pad 204. The signal-fiber array 50 may bepositioned, such that the end faces 56 of the optical fibers 52 extendslightly beyond the edge of the support substrate 20 (e.g., 100-200 μm).In some example embodiments, the signal-fiber array 50 may be retracted,such that the end faces 56 of the optical fibers 52 are flush with anedge of the support substrate 20, or even slightly inside the edge ofthe support substrate 20. Having the end faces 56 of the optical fibers52 flush with, or slightly inside, the edge of the support substrate 20may help with polishing, preventing the end faces 56 of the opticalfibers 52 from breaking off during initial polishing.

In the next step the signal-fiber array 50 may be squeezed together.Downward force is applied to the support substrate 20 to compress therelease pad 204 and force the signal-fiber array 50 into contact withthe surface of the support substrate 20. Simultaneously, the pushersheets 206 may be moved inward towards the release pad 204, such thatthey provide a squeezing force that forces adjacent optical fibers 52 ofthe signal-fiber array 50 into contact with each other. The forcesapplied to the release pad 204 may cause the release pad 204 to conformto the shape of the signal-fiber array 50, both on the exposed surfaceof the signal-fiber array 50 and on each of the edges, or sides. Whilethe squeezing forces are still being applied, the adhesive 70 may beilluminated by UV light through the support substrate 20. The UV lightmay pass through the glass, or other transparent, support substratecausing the adhesive 70 to cure and bond the optical fibers 52 of thesignal-fiber array 50 to each other and the support substrate 20.

After the adhesive 70 has been cured the pusher sheets 206 may beretracted to terminate the horizontal, or lateral, squeezing force onthe release pad 204. The vacuum tip 210 may also be removed from thesupport substrate 20 to reveal the signal-fiber array 50 in the straightchannel region SR of the release pad 204. In some example embodiments,the vacuum tip 210 remains attached to the support substrate 20 aftercuring of the adhesive 70. This may allow the support substrate 20 andthe attached signal-fiber array 50, e.g. the lidless FAU 10, to beremoved from the release pad 204. Since the adhesive 70 does not stickto the release pad 204, the FAU 10 may be easily removed from therelease pad 204.

FIGS. 14A and 14B illustrate a top and side view, respectively, of anFAU 10 including a strain relief adhesive 71. In some exampleembodiments, the strain relief adhesive 71, which may be the sameadhesive as adhesive 70, may be applied to the assembled FAU 10. Thestrain relief adhesive 71 may stabilize and/or protect the bare-glassfront-end sections 54 of the signal-fiber array in a region between thesupport substrate 20 and the multifiber cable 60. In some exampleembodiments, the top surface of the support substrate 20 may have a stepdown surface that may be disposed in a plane lower than the top surface.The bare-glass front-end sections 54 may be disposed on the top surfaceand the portion of the optical fiber having the protective coating maydisposed on the step down surface.

Turning to FIG. 15 , passive alignment of a lidless FAU 10 to a glass orsilicon waveguide substrate 300 may be achieved by etching a notchfeature 302 with precise surfaces disposed along straight or angledsidewalls into the waveguide substrate 300. Examples of waveguidesubstrates 300 include, without limitation, photonic integrated circuit(PIC) interposers and adapter substrates that may be used to interfacewith, e.g. optically connect to, a PIC substrate 400. In the depictedexample, a waveguide substrate 300 serves as an adapter for coupling theoptical fibers 52 of the signal-fiber array 50 to PIC planar waveguides404. The waveguide substrate 300 may include an evanescent couplingregion 306 including one or more laser written waveguides 304 In anexample embodiment, the notch feature 302 is formed by a precision laserdamage and etch process. The notch feature 302 may include a width sizedto match a width of the signal-fiber array 50 of the FAU 10. It is notedthat the depicted example provides a separate waveguide substrate andPIC substrate, however these may be the same substrate. Similarly, thewaveguide substrate may be one integral component or may have multiplecomponents that are attached to one another. In the discussion below,the term “waveguide substrate” should be understood to mean a substrateoptically connected to the PIC either directly or indirectly.

In some example embodiments, the notch feature 302 may be fabricated inthe same step as a PIC chip perimeter deep etch (used to exposewaveguides that terminate around the edge of the PIC), using the samemask layer and etch process. Therefore, no additional cost is requiredto add the notch feature 302 when PIC chip perimeter deep etching isused. Alternatively, the notch may be formed in a separate substratecomponent and attached to the waveguide substrate, such as by anadhesive.

The notch feature 302 may be positioned at the edge of the waveguidesubstrate 300, as shown in FIG. 15 . In another embodiment, the notchfeature 302 may be positioned toward the middle of the waveguidesubstrate 300, such that it appears as a well of sufficient length (andwidened as necessary) to allow the multifiber cable 60 of the FAU 10 tobend upward out of the well.

Passive alignment of the lidless FAU 10 to the notch feature 302 of thewaveguide substrate 300 provides an advantage over bare fiber arrayspassively aligned in V-grooves in that they are more compact, easier toprepare in advance, and more practical for use in passive alignmentpick-and-place applications. The vertical offset of the cores 72 of theoptical fibers 52 may also be more accurately controlled relative to thebottom surface of the support substrate 20.

FIG. 16 depicts lidless FAU 10 positioned over a U-shaped notch feature302 that is etched into the surface of a waveguide substrate 300. In anexample embodiment, the waveguide substrate 300 may be glass, and thenotch feature may be formed using a laser damage and etch process.Additionally, an ultra-short pulse laser process may be used to form anarray of buried waveguides 304 in the waveguide substrate 300. In someexample embodiment, the two laser forming processes may be performedusing the same laser, so that the laser written waveguides 304 areprecisely aligned to the left and right sidewall edges of the notchfeature. The exposed end faces of the laser written waveguides 304 mayalso be laser polished to create optically smooth surfaces to minimizecoupling losses.

In the depicted embodiment, the notch feature 302 includes chamferedsurfaces 314 where the notch feature 302 sidewalls meet a top surface ofthe waveguide substrate 300. The chamfered surfaces 314 may aid inpassive alignment of the FAU 10 with the notch feature 302. The width ofthe notch feature 302 may be sized to be slightly wider than the widthof the signal-fiber array 50 of the FAU 10 (by, for example, 0.5 μm).When the signal-fiber array 50 of the FAU 10 is inserted into the notchfeature 302, the cores 72 of the optical fibers 52 may be preciselyaligned with the cores of the laser written waveguides 304 in thewaveguide substrate 300. The alignment may be accomplished by formingthe laser written waveguides 304 at a precise depth in the waveguidesubstrate 300, such as exactly one half the diameter of the opticalfibers 52 (e.g., 62.5 μm for standard 125 μm diameter optical fiber).

The FAU 10 may be passively aligned laterally, to waveguides 304 of thewaveguide substrate 300 by contact of the sides of edge optical fiber 52(particularly the second datum surfaces 104) with precision surfaces 310formed at selected locations on the sidewalls of the notch feature 302.Generally, laser write time may be longer for forming precision surfaces310 than coarse or rough surfaces. To minimize fabrication time andcost, the area of the notch feature 302 that is dedicated to precisionsurfaces 310 is minimized. Other surfaces 312 of the notch feature 302,such as other locations on the notch sidewalls and bottom surface, maybe fabricated using a fast laser forming process that results in a roughsurface. The rough surface does not participate in passive alignment orserve as datum contact or surface. The precision of each of the passivealignment elements may enable lateral alignment of single mode waveguidecores to within <1 μm, and preferably to within <0.5 μm.

Prior to passive alignment and attachment of the FAU 10 to the waveguidesubstrate 300, UV-curable adhesive may be applied to the side walls ofthe notch feature 302 and/or the FAU 10, such as on the signal-fiberarray 50. After the FAU 10 is inserted into the notch feature 302, amoderate downward force may be applied on top side surface of FAU 10.This force may exclude adhesive from a region where the FAU 10 contactsthe top surface of the waveguide substrate 300. In the depicted example,vertical alignment of the cores 72 to the waveguides 304 may be providedby contact of the surface of the support substrate 20 of the FAU 10 withthe top surface of the waveguide substrate 300. Roughness of othersurfaces 312 may improve the bond strength of UV-curable adhesive 70used to bond the optical fibers 52 to the notch feature 302.

Advantageously, the laser write time for forming the notch feature 302may be significantly less than a comparable V-groove structure foraligning individual V-grooves due to fewer precision surfaces needed foroptical fiber alignment (two precision sidewall surfaces for all of theoptical fibers 52 of the signal-fiber array 50 vs. 2 N precisionV-groove surfaces for each optical fiber 52 of the signal-fiber array50). Additionally, there is significantly less bottom surface areaassociated with the notch feature 302 due to the optical fibers 52 beingarranged in a 125 μm pitch instead of the 250 μm pitch of similarV-groove structures.

In some fabrication processes of waveguide substrate 300, such as ionexchanged (IOX) waveguide fabrication, the waveguides 304 may be planarwaveguides that are formed near the surface of the waveguide substrate300. FIGS. 17A and 17B provide views of an etched notch feature 302 on awaveguide substrate 300 with an array of waveguides 304 fabricated nearthe top surface of the waveguide substrate 300. The notch feature 302provides two precision surfaces 310 on opposing sidewalls that extendupward to meet the top surface of the waveguide substrate 300.

Similar to the FAU 10 depicted in FIGS. 10A and 10B, a D-shaped opticalfiber 52A may be selected so that the distance between the core 72 andthe flat portion matches the depth of the waveguide 304 below the topsurface of the of the waveguide substrate 300 of the waveguide substrate300, for example the waveguide depth may be 5 to 10 μm. FIG. 17B shows alidless FAU 10 assembled using a signal-fiber array 50 having D-shapedoptical fibers 52A that has been inserted into the notch feature 302 ofthe waveguide substrate 300. The cores 72 of the D-shaped optical fibers52A align with the waveguides 304 disposed near the surface on thewaveguide substrate 300. Lateral alignment of the cores 72 to thewaveguides 304 may be achieved by the contact between the precisionsurfaces 310 of the notch feature 302 and the datum surfaces 104 of theedge optical fibers 52 of the signal-fiber array 50. The FAU 10 withD-shaped optical fibers 52A may be held in the notch feature 302 of thewaveguide substrate 300 using a UV-curable adhesive, as discussed above.

Some PIC planar waveguide technologies position waveguides at thesurface of the PIC substrate 400, or possibly only slightly below thesurface of the PIC substrate (e.g., 1-3 μm below). Examples includesilicon, silicon nitride, and silicon oxynitride, and polymer waveguidesfabricated as ridge waveguides or near-surface buried waveguides onsilicon, glass, and LiNbO3 substrates. These waveguides may be employedin passive or active waveguide components. To accommodate PIC substratesthat include surface waveguides, the FAU 10 may be modified to includeone or more vertical standoff features so that its depth in the etchednotch is precisely controlled. FIGS. 18A and 18B depict an example FAU10 embodiment including vertical standoff features.

Drawn glass fibers, e.g. spacer fibers, may be fabricated with precisediameters, often to within <0.5 μm of target values. Spacer fibers 450with precise diameters may be attached to the surface of supportsubstrate 20 of the FAU 10 adjacent to the signal-fiber array, asdepicted in FIG. 18A. The spacer fibers 450 may be attached to thesupport substrate 20 during or after the fiber array squeeze assemblyprocess. In some examples, spacer fibers 450 may also be utilized as thepusher sheets 206, discussed above. The spacer fibers 450 may push onthe opposing sides of the signal-fiber array 50 during assembly to pushthe optical fibers 52 into contact with each other. Excess length of thespacer fiber 450 may be removed after assembly of the FAU 10, such asvia scoring and cleaving, or sawing. When the signal-fiber array 50 ofthe FAU 10 is inserted into the notch feature 302 of the waveguidesubstrate 300 the spacer fiber 450 provides a precise vertical spacing,such that the cores 72 are aligned to the planar waveguides 304 of thewaveguide substrate 300. Lateral alignment of the cores 72 to thewaveguides 304 may be achieved by the contact between the precisionsurfaces 310 of the notch feature 302 and the datum surfaces 104 of theedge optical fibers 52 of the signal-fiber array 50.

In the depicted embodiment, the spacer fibers 450 are shownperpendicular to longitudinal axis of the optical fibers 52. However, inan alternative configuration, the spacer fibers 450 may be arrangedparallel to the longitudinal axis of the optical fibers 52, as depictedin FIG. 18B. Additionally, in some example embodiments, the release pad204 may include small step recesses to accommodate two outboard smallerdiameter spacer fibers 450 during assembly of the FAU 10. The spacerfibers 450 may be squeezed and held in place by the adhesive 70, similarto the manner discussed above with to assembly of the FAU 10.

In further example embodiments, instead of using precision diameterspacer fibers 450, fusion drawn glass sheets may be used both as thepusher sheet 206 and as precision vertical spacer sheets. The assemblyprocess of the FAU 10 may be substantially similar to the FAU 10assembly process discussed above with regard to the spacer fibers 450.Excess length of the spacer/pusher sheet may be removed via glassscoring and breaking or laser scoring or ablation processes. When FAU 10is inserted into the notch feature 302 of the waveguide substrate 300,the spacer/pusher sheets provide a precise vertical offset that enablesalignment of the cores 72 of the optical fibers 52 of the signal-fiberarray 50 to the waveguides 304 of the waveguide substrate 300. Lateralalignment of the cores 72 to the waveguides 304 may be achieved by thecontact between the precision surfaces 310 of the notch feature 302 andthe datum surfaces 104 of the edge optical fibers 52 of the signal-fiberarray 50.

Additionally or alternatively, laser bumps may be formed on the supportsubstrate 20 and/or the waveguide substrate 300 with precise heights.For example, laser bumps that are 2 to 100 μm high may be formed onsurfaces of the support substrate 20 and/or waveguide substrate 300 witha height control of <0.5 μm. Using laser bumps to control core depthalignment, three or more laser bumps may be formed on the surface of thesupport substrate 20 adjacent to the signal-fiber array 50 and/or thesurface of the waveguide substrate 300 adjacent to the notch feature302. The bump heights may be selected, such that when the FAU 10 isinserted into the notch feature 302, the cores 72 of the optical fibers52 are aligned with the waveguides 304.

FIG. 18A depicts an example embodiment, in which the notch feature 302includes straight sidewalls etched in to the waveguide substrate 300.The notch feature 302 may be etched, as discussed above, using laserdamage and etch processes in waveguide substrates 300 formed from glass.In other embodiments, the notch features 302 may be formed in waveguidesubstrates 300 from silicon using anisotropic deep RIE (Reactive IonEtch) processes, such as the Bosch process. The anisotropic deep RIEprocess may be tuned to create deep etches with smooth verticalsidewalls. An example etch depth using the anisotropic deep RIE processwith FAUs 10 that have round optical fibers may be about 70 μm. Thedepth of the notch feature 302 does not have to be precisely controlled,however the width of the notch feature 302, particularly the precisionsurfaces 310 may precisely controlled via photolithographic etchmasking.

FIG. 18B depicts a silicon waveguide substrate 300 including a notchfeature 302 formed using a combination of isotropic and anisotropic etchprocesses. The notch feature 302 may be formed in three steps, suchas 1) shallow anisotropic etch to form a precision surface 310 disposedon the side wall of the notch feature 302; 2) sidewall passivation toprevent etching at precision surface in a subsequent step; and 3) deepisotropic etch to rapidly remove material moving downward into waveguidesubstrate 300. The combination of isotropic and anisotropic etchprocesses mimics the low cost and rapid SCREAM (Single-Crystal ReactiveEtching And Metallization) process used to produce MEMS(Micro-Electro-Mechanical Sensor) sensors and accelerometers viaunderetch. Advantageously, the combination of isotropic and anisotropicetch processes produces the required precision surface 310 of the notchfeature 302, while allowing the remaining surfaces to be etched awayrapidly.

The sidewall underetch may also be used to create a notch feature 302that holds the FAU 10 captive after passive alignment to the siliconwaveguide substrate 300, as depicted in FIG. 19 . Similar notch features302 may also be fabricated in glass waveguide substrates 300 using laserdamage and etch processes. A waveguide substrate 300 may include a notchfeature 302 with a sidewall having an undercut with a precision surface310. The undercut profile may be etched such that the precision edgematches the outer diameter of the optical fibers 52 of the signal-fiberarray 50 of the FAU 10, enabling contact between the undercut profileand the optical fibers 52 at at least one point.

The notch feature 302 may be tapered along its length (parallel to theaxis of optical fibers 52 of the signal-fiber array 50), such thatduring insertion of the FAU 10 into notch feature 302, the undercutfeatures gradually engage the outer diameter surface of the opticalfibers 52. When the FAU 10 is fully inserted in the notch feature 302,the clearance between the precision surface 310 of the undercut and thesurface of the outer diameter of the optical fiber 52 may be about 0.5μm. This clearance may facilitate low loss coupling between cores 72 ofthe optical fibers 52 and the waveguides 304 of the waveguide substrate300.

Additionally or alternatively, the sidewalls and/or the precisionsurfaces 310 of the notch feature 302 may also be angled usinganisotropic wet etch processes, forming a broad V-groove notch feature.In such an example embodiment, laser formed glass bumps may be providedon the support substrate 20 of the FAU 10 to ensure that the FAU 10 isproperly aligned in the broad V-groove notch feature and cores 72 of theoptical fibers 52 are aligned to the waveguides of the waveguidesubstrate 300.

One solution for coupling an FAU 10 to a waveguide substrate 300 withsurface waveguides 304 may be to include vertical offset features on theFAU 10, such as laser bumps, fiber spacers, or sheet spacers, asdescribed above. Another approach may include setting the verticaloffset of the FAU 10 by fabricating bottom precision surfaces 311 on thebottom of the notch feature 302. The sidewall precision surface 310 andthe bottom precision surfaces 311 may be fabricated in glass waveguidesubstrates 300 using laser damage and etch process. Lateral alignment ofthe cores 72 to the waveguides 304 may be achieved by the contactbetween the precision surfaces 310 of the notch feature 302 and thedatum surfaces 104 of the edge optical fibers 52 of the signal-fiberarray 50. Vertical alignment of the cores 72 to the waveguides 304 maybe achieved by the contact between the bottom precision surfaces 311 ofthe notch feature 302 and the datum surfaces 103 on the top of one ormore of the optical fibers 52 of the signal-fiber array 50.

Additionally or alternatively, passive alignment of the FAU 10 with thenotch feature 302 of the waveguide substrate 300 may be accomplished byproviding a single alignment feature 308 on the surface of the waveguidesubstrate 300 that serves as a lateral stop, as depicted in FIG. 21 .The alignment feature 308, or rib, may be fabricated on the waveguidesubstrate 300 using a photolithographic or laser processing step, inwhich the alignment feature 308 is positioned at a predetermined lateraloffset from laser written waveguides 304. The alignment feature 308 mayinclude a precision surface disposed on an edge facing the notch feature302. The alignment feature 308 may also be a thin glass substrate with aprecision edge facing the FAU 10 that is bonded to the waveguidesubstrate 300 using adhesive. In such an embodiment, the alignmentfeature 308 may be actively aligned to the waveguide substrate 300 withsubmicrometer precision using, for example, fiducial marks provided onboth the alignment feature 308 and the waveguide substrate 300.

The FAU 10 may be attached to the waveguide substrate 300 where the FAU10 is positioned against the alignment feature 308. In this approach,each core 72 of the signal-fiber array 50 of the FAU 10 is positioned onthe support substrate 20 at a precise lateral position relative to theedge of the support substrate 20. An axial force F1 may be applied toinsert the FAU into the notch feature 302, while a lateral force F2 isapplied to the FAU against the alignment feature 308. A concern withthis approach is that dicing processes for cutting large glass sheetsinto smaller support substrates 20 may not create a precise straightedge, such that the edge may have some waviness, roughness, edgechipping or other deviation from an ideal straight line along its lengththat is on the order of 1 μm or more.

To overcome inconsistency in the edge of the support substrate 20, laserdamage and etch processes may be used to fabricate a precision surfaceon selected perimeter locations of the support substrate 20. In anexample embodiment, a large glass sheet is first patterned by aultra-short pulse laser to create a precision rectangular notch adjacentto each glass support substrate 20. The notch may be formed all the waythrough the glass sheet or etched to a sufficient depth to enableassembly using the lidless FAU squeeze approach, described above inreference to FIGS. 3-10B.

After the notch is etched away the glass sheet may be cut into supportsubstrates 20 using a dicing saw. Alternatively, the laser damage andetch process may be used to form both the precision rectangular notchesand less precise straight cuts that separate the support substrates 20,using a faster laser exposure process that leaves a more coarse surfaceon the edges of the support substrate 20 after etching.

FIG. 22 provides an end face view of an FAU 10 with a precision edgesurface during assembly. During assembly of the FAU 10 an edge 23including the precision surface of the support substrate 20 may be havea precise lateral offset (d′) to the datum surface 104 of the edgeoptical fiber 52 of the signal-fiber array 50. A stepped pusher sheet206A may be positioned on the edge 23 of the support substrate 20 andmay maintain the lateral offset d′ during the squeeze assembly processof the FAU 10. The stepped pusher sheet 206A may be fabricated using adiamond turning process to provide the precise lateral offset d′. Thestepped pusher sheet 206A may also be coated with a non-stick coating,such as a fluorosilane coating, such that after curing of the adhesive70, the FAU 10 does not become attached to the stepped pusher sheet206A.

Lidless FAU mounting to Flip Chip PIC

An emerging trend in high bandwidth data center switches involvesco-location of many compact optoelectronic transceivers aroundelectronic switch chips on a common interposer substrate or multi-chipmodule. Flip chip mounted PICs are preferred because they enable highbandwidth solder bump electrical interconnections to supportinginterposer or PCB substrates, and they are compatible with low cost pickand place assembly techniques.

A challenge with flip chip mounting of PICs is that theirphotolighographically defined features, such as waveguides, alignmentfeatures, alignment marks, and control electronics, are located on theside of the PIC that faces downward toward the supporting interposer orPCB substrate. This configuration makes it difficult to access orobserve these features and use them for passive alignment of opticalinterconnections. As a result most PICs are mounted with theirwaveguides and other features facing upward, requiring more complicatedpackaging and electrical interconnection solutions.

Laser damage and etch processes may be used to rapidly form precisionfeature on glass sheets. Etched features may be positioned with highprecision (e.g., <0.5 μm) relative to other etched features, laserwritten waveguides, or surface or edge datum features. Glass substrateswith precision features may be attached to PIC substrates since theglass substrate CTE (Coefficient of Thermal Expansion) may be adjustedto match the PIC substrate CTE. This may enable a fully passivealignment assembly approach for coupling lidless squeeze FAUs (to flipchip mounted PICs via an intermediate glass alignment substrates, wherethe glass alignment substrates are patterned with precision laser damageand etch channel features. Although discussed below in the context offlip chip mounted PICs, the described method and structures may also beapplied to any configuration that the PIC waveguides and alignment marksare not easily accessible after the PIC is mounted on a supportincluding without limitation, a PCB, another PIC, an IC, or the like,alone or in a 3D stack.

Turning to FIGS. 23 and 24 , a method for passively aligning a lidlessFAU 10 to a PIC substrate 600 with flip chip mounting is provided. ThePIC substrate 600 may include one or more electric connection elements,such as solder pads, disposed on a PIC face 601. The PIC substrate 600may be flip chip mounted to a printed circuit board (PCB) 603 or otherelectronic packaging support substrate, such as a glass or siliconinterposer substrate. The PCB 603 may include one or more PCB electronicconnection elements, such as solder pads, which may be complementary tothe at least some of the electronic connection elements of the PICsubstrate 600. The PCB electronic connection elements and the electronicconnection elements of the PIC substrate 600 may be connected by one ormore solder balls 605, using a solder reflow process. A glass alignmentsubstrate 602 may be mounted to the bottom surface, e.g. PIC face 601,of the PIC substrate 600. The alignment substrate 602 may include aprecision channel, such as a U-channel 604 or V-channel, that enablespassive alignment of the lidless FAU 10 to the PIC substrate 600. Forexample, the PIC substrate 600 may include one or more alignmentfeatures, such as alignment ribs 606 extending from the PIC face andconfigured to engage the U-channel 604 to align the alignment substrate602 with one or more planer waveguide 608 disposed on the PIC face 601of the PIC substrate 600. Additionally, the external features, e.g.exposed datum surfaces 103, 104, as discussed above in reference to FIG.2 may also engage the U-channel 604 to align the signal-fiber array 50with the waveguides 608. Additionally or alternatively, the alignmentfeature on the PIC face 601 may be configured to engage a separatechannel, e.g. U-groove or V-groove, disposed on the alignment substrate602, where the separate channel is formed at a precision offset from theU-channel 604 used for alignment to the plurality of waveguides 608. Insome example embodiments, the alignment feature extending from the PICface 601 may be configured to align with a precision edge on thealignment substrate 602.

In some example embodiments, a recess 607 may be provided in the PCB603. The recess 607 may enable the PIC substrate 600 to be mounted to atop face of the PCB and allow at least a portion of the alignmentsubstrate 602 to be disposed below the top face of the PCB 603. Bypositioning at least a portion of the alignment substrate 602 below thetop face of the PCB 603 the signal fiber array 50 may be aligned withthe waveguides 608, as shown in FIG. 24 .

The lidless FAU 10 may be passively aligned to the alignment substrate602 and then held in place using UV-curable adhesive. The process asdescribed in FIGS. 23-34C may include fabrication of the alignmentsubstrate 602, passive alignment of alignment substrate 602 to a PICsubstrate 600, flip chip attachment of PIC substrate 600 to PCB 603,passive alignment of the lidless FAU 10 to PIC waveguides 608,attachment of the lidless FAU 10 using UV-curable adhesive, and/orretaining lidless FAU 10 in contact with PIC substrate 600, as discussedbelow.

Turning to FIGS. 25A and 25B, the U-channel 604 of the alignmentsubstrate 602 may be formed by laser damage and etch processes in aglass substrate. The U-channels 604 may include characteristics similarto the notch features 302, as discussed above, such as one or both ofthe width and the depth of the U-channel 604 may be precisely controlledand/or include a precision surface.

The alignment substrate 602 may be fabricated starting with a glasssheet that provides an extremely flat surface, such as standard LiquidCrystal Display (LCD) glass. Precision width U-channels 604 may beformed into the surface of the glass sheet using laser damage and etchprocesses, precision photolithographic masking and etching, or precisiondiamond sawing. The widths of the U-channel 604 slot may be configuredto be only slightly wider than the width of the signal-fiber array 50 ofthe lidless FAU 10, for example 0.5-0.7 μm wider than the signal-fiberarray 50. The U-channels 604 may also be fabricated with a precise depthsuch that when the signal-fiber array 50 is passively aligned by theU-channel 604, the cores 72 (FIG. 1 ) are vertically, as well ashorizontally, aligned to waveguides 608 of the PIC substrate 600.

As depicted in FIG. 25B, the glass sheet may be separated into smallerpieces, e.g. alignment substrates 602, using a dicing saw, as indicatedby dashed lines 611. Alternatively, the augment substrate 602 may beseparated using a fast laser damage and etch process. After dicing,individual alignment substrates 602 may be ready for passive alignmentto a PIC face 601 of a PIC substrate 600. Depending on the applicationand the number of optical interconnections required by the PIC substrate600, the alignment substrate 602 may include one U-channel 604, or itmay include many U-channels 604 arranged side-by-side.

FIGS. 26A-26C depict bottom, side, and end views of passive alignment ofthe alignment substrate 602 to the PIC substrate 600 using precisionalignment features, such as the raised alignment ribs 606. The alignmentribs 606 may be precisely positioned on the PIC face 601 of the PICsubstrate 600 relative to the waveguides 608 using, for example,photolithographic alignment techniques. The alignment ribs 606 may bepositioned, such that the distance between outside surfaces of thealignment ribs 606 matches the width (W) of the U-channel 604 ofalignment substrate 602. A slight clearance tolerance may be provided toaccommodate manufacturing variations of the alignment ribs 606 and thewidth of the U-channel 604, such as 0.2-0.5 μm. FIGS. 27A-27C depict anoptical assembly after passive alignment the alignment substrate 602 andthe PIC substrate 600, where UV-curable adhesive is used to hold the twocomponents together.

The alignment features, such as alignment ribs 606 may be fabricatedusing a variety of techniques for glass, silicon, LiNBO3, and otheroptoelectronic substrate materials. Non-limiting example techniques forfabricating raised alignment ribs, blocks, or posts using additiveprocesses may include photoresist, metal, or other deposited materials.Non-limiting example techniques for fabricating raised alignment ribs,blocks, or posts by removing substrate material adjacent to the ribs ordepressed surfaces may include slots, channels, grooves, or pits formedby reactive ion etch (RIE) processes, wet chemical etch processes, laserdamage and etch processes (for glass substrates). The raised alignmentribs may also be created by adding a precision geometry object, such asan optical fiber into a precision depressed surface, such as a V-groove.The U-channel 604 edges on the alignment substrate 602 may be formedwith sharp corners, therefore, the alignment ribs 606 do not have to beextremely high. For example, alignment of the U-channel 604 may beachieved with a height of the alignment feature of 5-10 μm.

Additional lead-in features, such as tapers and funnels, may beincorporated into the shape of the alignment rib 606 or the U-channel604 of the alignment substrate 602 to aid in initial coarse alignment(both lateral and angular) of the U-channel 604 to the alignment ribs606 of the PIC substrate 600. The alignment rib 606 may also include aperpendicular feature, extending out of a longitudinal axis of thealignment rib 606, that serves as a stop to arrest the motion of theglass alignment substrate during alignment. Alternatively the alignmentsubstrate 602 may include a top surface step feature that causes thealignment substrate 602 to stop at a precise location relative to theedge of the PIC substrate 600 during passive alignment. In someembodiments, the outside edge of the alignment substrate 602 may also beused as a passive alignment datum, such as when it is formed using aprecision etching process. The laser damage and etch process may also beused to create additional alignment features on the alignment substrate602, such as trenches, ribs, posts, or notches that are designed toalign to complementary mating features on the PIC face 601 of the PICsubstrate 600. A number of options may be utilized for the passivealignment assembly process. Some non-limiting examples include passivealignment carried out between individual alignment substrates 602 andindividual PIC substrates 600, as depicted in FIGS. 26A-27C; oralignment substrates 602 fabricated in strips with 1D arrays ofU-channels 604, where the multiple U-channels 604 may be passivelyaligned to an individual PIC substrate 600. In a further example,alignment substrates 602 may be fabricated in strips with 1D arrays ofU-channels 604 for passive alignment to 1D strips of multiple PICsubstrates. After passive alignment the joined components are diced toproduce individual alignment substrate 602/PIC substrate 600 assemblies.Another example assembly process may include wafer-scale assembly, wherea 2D array of U-channels 604 of alignment substrate 603 are aligned on asingle glass sheet and are passively aligned to a 2D array of PICsubstrates 600 on a single wafer. After passive alignment the joinedcomponents are diced to produce individual alignment substrate 602/PICsubstrate 600 assemblies.

After assembly, a protective material (e.g., tape or film material) maybe applied over the U-channel 604 of the alignment substrate 602 toprotect the U-channel 604 from contamination during subsequentprocessing. The protective material may be made of a material configuredto survive exposure to solder reflow temperatures (−260° C. for severalminutes), such as Kapton.

As discussed above in reference to FIGS. 23 and 24 , the alignmentsubstrate 602/PIC substrate 600 assembly may be attached to the PCB 603using flip chip solder ball or solder bump joining processes. Thealignment substrate 602 and the PIC substrate 600 are joined using anadhesive that is solder reflow compatible. The alignment ribs 606 engagethe U-channels 604 of the alignment substrate 602 to stabilize thealignment even if the adhesive softens during solder reflow process. Anadvantage of this approach is that during solder reflow the lidless FAU10 is not connected to the alignment substrate 602, so applied forcesthat may misalign the alignment substrate 602, relative to the PICsubstrate, 600 may be minimized.

The applied protective high temperature tape that covers over theU-channel 604 of the alignment substrate 602 may be removed after thePIC substrate 600 is attached to the PCB 603 or interposer by the solderreflow process. Alternatively, the tape covering the U-channel 604 ofthe alignment substrate 602 may be left in place until subsequentpassive alignment of the lidless FAU 10 with the U-channel 604.

Next, the lidless FAU 10 may be passively aligned with flip chip mountedPIC substrate 600 by inserting the lidless FAU 10 into the U-channel604. The precision surfaces of the U-channel 604 may engage the exposeddatum surfaces 103, 104 of the lidless FAU 10, thereby aligning thesignal-fiber array 50 with the waveguides 608. In an example embodiment,the lidless FAU 10 may be similar to the FAUs discussed above inreference to FIG. 2 . In some example embodiments, an overlap sheet 612may be disposed on a second surface of the support substrate 10,opposite the signal-fiber array 50, as depicted in FIGS. 28A and 28B.

The overlap sheet 612 may be an enlarged glass sheet that is configuredto overlap the PIC substrate 600 during assembly to increase themechanical strength of the interface. The enlarged glass sheet, e.g.overlap sheet 612, may be installed over a lidless FAU 10 to form anoverlap FAU, as depicted in FIG. 28B. An adhesive, such as a UV curableadhesive, may be utilized to bond the overlap sheet 612 to the supportsubstrate 10. In this configuration, the thickness of support substrate10 may be selected such that the bottom of the overlap sheet 612 isapproximately even to a top surface of the PIC substrate 600 afterpassive alignment. The thickness of the support substrate (T_(SS)) mayfound using EQN. 1T _(SS) =T _(PIC) −r _(OF) −D _(WG) +T _(AD) +Var  EQN. 1where T_(PIC) is the thickness of the PIC substrate, r_(OF) is theradius of the optical fiber, D_(WG) is the depth of the PIC waveguidecenter below the surface of the PIC substrate; T_(AD) is the targetnominal adhesive thickness between the bottom of the overlap glass sheetand the top of the PIC substrate, and Var is an additional thickness toaccount for variations in the thickness of the PIC substrate.Additionally or alternatively, spacers features may be added to thebottom surface of the overlap sheet 612 to relax the tolerances on thethickness of the support substrate 10. The spacers may be laser bumps,precision fibers, fiber array rafts, glass spacer sheets, or the like.

FIGS. 29A-29C depict top, side and end views of a passive alignment ofsignal-fiber array 50 of the lidless FAU 10, including the overlap sheet612, to the waveguides 608 of the PIC substrate 600. As the lidless FAU10 is inserted into the U-channel 604, the curved fiber sidewall profileof the signal-fiber array 50 may guide the signal-fiber array 50 intothe U-channel 604. After passive alignment the lidless FAU 10 contactsthe alignment substrate 602, such that the signal-fiber array 50 of thelidless FAU 10 is aligned by the U-channels 604 of the alignmentsubstrate 602, as depicted in FIGS. 30A-30C. At least a portion of theoverlap sheet 612 extends past a forward edge of the Lidless FAU,defined by the ends of the signal-fiber array 50, such that the portionof the overlap sheet 612 covers a portion of the PIC substrate 600. Alayer of adhesive 614, such as UV-curable adhesive, may be appliedbetween a bottom surface of the portion of the overlap sheet and a topsurface of the portion of the PIC substrate 600, which may also becompressed during assembly. The adhesive 614 may also be applied in theU-channel of the alignment substrate 602, such that signal-fiber array50 of the lidless FAU 10 is bonded to the alignment substrate, therebyproviding additional strength to the assembly. UV exposure to theadhesive 614 assisted by the transparency of the support substrate 10and overlap sheet 612.

In an example embodiment, a force may be applied to the top of thelidless FAU 10 to assist in retention of the lidless FAU 10 and maintainaxial alignment of the signal-fiber array 50 with the waveguides 608. Inthe example depicted in FIG. 31 , a cap 616 is disposed on a top surfaceof the PIC substrate 600. The cap 616 may extend from the PIC substrateover a portion of the alignment substrate 602 to form a receiving area.Similar to the overlap sheet 612, discussed in reference to FIGS.29A-30C, the cap may be bonded to the top surface of the PIC substrate600 by an adhesive 614. In some example embodiments, a retention featuremay be disposed on one or both of the cap 616 and the support substrate10 to resist removal of the lidless FAU from the U-channel 604. In thedepicted embodiment, a protrusion feature 618 is disposed on the cap 616that engages a mating depression 620 formed on a top surface of thesupport substrate 10.

In another example embodiment depicted in FIGS. 32A-32C, the lidless FAU10 may be retained using a clip 622 configured to wrap around the PICsubstrate 600. The clip 622 may include a catch or grip feature 624,such as a projection or tab, configured to engage with retaining notchfeatures 626 formed in the support substrate 10 of the lidless FAU 10.The notches 626 of the support substrate 10 may be formed using laserdamage and etch processes or glass pressing or notch sawing processes.

The clip 622 may be formed from metal, such as steel or aluminum or maybe formed from plastic, such as an injection molded plastic. The clip622 may include a vertical arm 628 extending across an edge of the PICsubstrate from the top surface of the PIC substrate 600 to the PIC face601. The vertical arm 628 may be configured to retain the clip 622 incontact with the PIC substrate 600. A fixture element 630 may extendfrom the vertical arm 628 and across a portion of the PIC face 610 and,in some embodiments, include a substrate catch 632 configured to engagea backside of the alignment substrate 602. A horizontal arm of the gripfeature 634 may extend over a portion of the top surface of the PICsubstrate 600, such that the clip wraps around the edge of the PICsubstrate 600. The clip 622 may provide axial force to hold thesignal-fiber array 50 of the lidless FAU 10 in contact with thewaveguides 608 of the PIC substrate 600. The grip feature 634 may alsoextend outward from the edge of the PIC substrate 600. The grip feature634 may be configured to provide a force to bias the support substrate10 of the lidless FAU 10 toward the U-channel 604, similar to the cap616 discussed in reference to FIG. 31 . In an alternative embodiment,the clip 622 may be configured to engage a distal edge, or backside, ofthe support substrate 10 of the lidless FAU 10, which may eliminate aneed to form notches 626 in the support substrate 10. FIGS. 33A-33Cdepict the lidless FAU 10 installed into the U-channel 604 and retainedtherein by the clip 622.

Turning to FIGS. 34A-34C, the clip 622 may be positioned, opened, andclosed via a dual-arm tweezer-like actuator arm or “gripper” 640. Thearms of gripper 640 may separate from each other, causing the jaws ofthe metal clip, e.g. grip features 634 to open, as depicted by arrow A.This allows the lidless FAU 10 to be inserted into the U-channel 604 andattached to the PIC substrate 600. The opened grip features 634 of theclip 622 may then engage the lidless FAU 10. As the arms of the gripper640 are brought closer to each other the grip features 634 of the clip622 may apply force on the lidless FAU 10 to force the signal-fiberarray 50 into contact with the PIC substrate 600, or more particularly,the waveguides 608, and hold the lidless FAU 10 in position duringsubsequent product deployment.

The lidless FAU mounting discussed in reference to FIGS. 23-34C mayenable optical interconnections to flip chip mounted optoelectroniccomponents and PIC substrates after solder reflow processing, avoidingconnector signal-fiber array shifts due to adhesive softening, and/orrelaxing high temperature requirements on optical jumpers andconnectors. Utilizing glass lidless squeeze FAU and glass alignmentsubstrate that are CTE matched to silicon or other PIC substratematerials may minimize waveguide coupling misalignments in thermalcycling. The increased bonding area provided by embodiments including anoverlap sheet, a cap or a clip may enable more mechanically robustinterconnections than FAUs bonded to the PIC edge. Passive alignment ofthe lidless FAU to waveguides of the flip chip mounted PIC substrate maybe provided by an alignment substrate that passively aligns to PIC facealignment features. The lidless FAU mounting may also enable highdensity optical interconnections to PIC substrates, on 125 μm fiberpitch for standard SMF fiber, and with smaller pitches possible usingsmaller cladding diameter fiber.

Large Array FAUs

When scaling fiber array squeeze assemblies for lidless or lidded FAUslarge arrays (e.g., 24-96 fibers or larger) variations in fiber claddingdiameters may accumulate as large fiber arrays are squeezed together.These accumulated variations in cladding diameter may lead tounacceptable fiber array core position errors. However, the coreposition error may be minimized during the fabrication of large arrayFAUs by optimizing the order of the optical fibers, and associatedcladding variance prior to ribbonization.

Individual spools of single mode optical fiber (SMF) may be ribbonizedto form a multifiber cable 60. A plurality of optical fibers 52 may beconducted side-by-side through a coating arrangement that surrounds theoptical fibers with a material matrix and/or protective cable jacket 61.One such ribbonization process is disclosed in U.S. Pat. No. 5,486,378,entitled Method and Apparatus for Manufacturing an Optical RibbonConductor, filed on Aug. 11, 1993, which is incorporated herein in itsentirety. Another example ribbonization process is disclosed in U.S.Pat. No. 10,175,436, entitled Optical Fiber Ribbons and Ribbon MatrixMaterials having Low Oligamer Content, filed Jun. 15, 2016, which isalso incorporated herein in its entirety.

FIG. 35 illustrates a comparison of an ideal optical fiber 52 (A) and anoptical fiber 52 (B) with core and cladding variation. In the idealoptical fiber 52 (A), the diameter of the cladding 74 is a perfectcircle exactly 125 μm in diameter and the center of the core 72 ispositioned in the geometric center of the optical fiber 52 (as definedby the shape of the cladding 74). However, core position and claddingdiameter variations may be created as a function of the fiber drawpreform geometry and draw conditions, resulting in the optical fiber 52(B) having a variation in diameter and core center position. Thesevariations may result in the center of the core 72 being offset from thegeometric center of the optical fiber 52, creating what is commonlyreferred to as “core-to-cladding eccentricity” or “fiber coreeccentricity” (FCE). The offset may be characterized by both directionand magnitude. The magnitude of FCE is commonly referred to as FCEerror.

Regarding optical fiber cladding diameter variation, as the opticalfiber 52 is drawn a control system that manages cladding diameter viatension control is typically able to control with a 1 sigma variation of0.05 μm around a mean diameter of 125.0 μm. The diameter of the cladding74 varies slowly along the length of the optical fiber 52, withpeak-to-peak cyclic variations over relatively long fiber lengths(e.g., >10 meters). Some fiber preforms yield optical fibers 52 withperiodic fiber upsets, where the diameter may increase in local areas by0.3-0.5 μm. These upsets are characterized during the fiber drawprocess, with most spools having few to no upsets, and some spoolshaving large numbers of upsets. The FAUs 10 discussed above utilizefiber spools with few or no upsets to increase the precision alignmentof the cores 72 of the FAU to the waveguides 304 of the waveguidesubstrate 300. Fiber cladding diameter measurement systems on differentfiber draw towers may experience variations in measured values due tovariations in calibration. The calibration error may have a 1 sigmavariation of 0.05 μm. In some example embodiments, this source of errormay be eliminated by characterizing fiber cladding diameters immediatelyin advance of assembly of the FAU 10. Regarding core variation, fibercore eccentricity (FCE) may be approximately 0.1-0.2 μm, with a maximumof 0.5 μm based on screening measurements performed on the opticalfibers 52. In some example embodiments, the spools of optical fiber 52may enable a measurement of the diameter of the cladding 74 of theoptical fiber 52 at either end of the spool. Additionally, in someembodiments, cladding measurements performed during the draw process maybe available enabling an estimated average diameter of the cladding 74for each spool of optical fiber 52. FCE may also be considered and maybe added as a shift to the position of the core 72 after the diametervariations of the optical fiber have been determined. For example theFCE of an optical fiber may be measured, such as by imaging an end face56 of the optical fiber 52 and performing a sub-micron inferentialmeasurement, and the value of FCE applied to the length of the opticalfiber 52.

FIG. 36 shows a comparison of a first FAU 10 (A) fabricated with asignal-fiber array 50 including optical fibers 52 having a uniformdiameter and a second FAU 10 (B) fabricated with a signal-fiber array 50that has realistic variations in the diameter of the optical fibers 52.FIG. 36 also includes a grid overlay 80 with crosshatches 81representing the target fiber core center positions (“ideal fiber corepositions”) along a straight line with pitch (P). As may be seen inrelation to the first FAU 10 (A), the grid overlay 80 and the ideal corepositions represented by crosshatches 81 are based on ideal opticalfibers 52 (A) (see FIG. 3 ) being arranged side-by-side in an array.There is no fiber core position error in that ideal situation. For thesecond FAU 10 (B) that includes realistic variations in the diameter ofthe optical fibers 52, fiber core position error is determined by firstpositioning the grid overlay 80 mathematically, such that the gridoverlay 80 is centered on the midpoint of a line extending between thecenters of the cores 72 of the two outboard optical fibers 52.

With the grid overlay 80 in position, the fiber core position error maybe determined by measuring the distance between a given center of thecore 72 of the optical fiber 52 and a corresponding ideal gridcrosshatch 81, as shown in FIG. 37 . The fiber core position (FCP) errormay be found using EQN. 2FCP Error=(Error X ²+Error Y ²)^(1/2)  EQN. 2where Error X is distance between the center of the core 72 of theoptical fiber 52 and the corresponding crosshatch 81 of the grid overlay80 measured parallel to the X-axis; Error Y is distance between thecenter of the core 72 of the optical fiber 52 and the correspondingcrosshatch 81 of the grid overlay 80 measured parallel to the Y-axis;and FCP Error is center of the core 72 of the optical fiber 52 and thecorresponding crosshatch 81 of the grid overlay 80 measured along thestraight line path between them.

The signal-fiber array 50 of the FAU 10 may have different numbers ofoptical fibers 52 including, but not limited to, eight (8), sixteen(16), thirty-two (32), forty-eight (48), seventy-two (72), or ninety-six(96) optical fibers 52 to provide different sizes. FIG. 38 illustratesan example FAU 10 including 8 optical fibers 52. For each size thesignal-fiber array 50, the desired number of optical fibers 52 (e.g., Noptical fibers 52) may be selected at random from a large pool ofoptical fibers 52 with statistical estimates for a diameter variation ofthe cladding 74. The optical fibers 52 may be arranged in random orderon a support substrate 20 with each optical fiber 52 making contact witha neighboring optical fiber 52 at the datum contact 101, as discussedabove.

FIG. 39 illustrates a data plot 120 of one thousand (1000) random FAU 10configurations for different signal-fiber array sizes including eight(8), sixteen (16), thirty-two (32), forty-eight (48), seventy-two (72),or ninety-six (96) optical fibers 52. The data plot 120 includes themaximum fiber core position error on the Y axis and the FAUconfiguration number on the X axis. The FAU configurations have beensorted from lowest to highest maximum fiber core position error. As thesize of the signal-fiber array 50 increases, the maximum fiber coreposition error also increases. The maximum fiber core position errorwhen FCE included is larger than the maximum fiber core position errorfor optical fibers without FCE consideration.

The data plot 120 may also be used to generate a statistical estimatefor process yield. For example, the plot shows that 90% of the simulatedvalues for maximum fiber core position error were less than or equal to0.57 μm for a 32-fiber array with FCE considered. The data plot 120 mayalso be used to construct a plot of maximum fiber core position errorvs. size of the signal-fiber array 50 for a given target process yield.For example, at 90% yield the following curve fit was generated based onEQN. 3.Maximum fiber core position error (μm)=0.0579*N ^(0.6052)+σ_(FCE)  EQN.3

FIG. 40 provides a data plot 122 of maximum fiber core position error onthe Y axis vs. size of the signal-fiber array 50 for 90% target processyield. The data plot 122 enables rapid estimation of maximum fiber coreposition error for a range of sizes of the signal-fiber array 50. Thefigure also shows commercial FAU specifications as black squares.Additionally, the data plot indicates that lidless FAUs 10 may befabricated with maximum fiber core position errors that are less than orcomparable to maximum fiber core position errors associated with typicalV-groove-based FAUs.

Changes to the orderings of the same N optical fibers 52 in asignal-fiber array 50 may yield different values for maximum fiber coreposition error. In an example embodiment, a method is provided toidentify specific optimal orderings of the optical fibers 52 thatreduces the maximum fiber core position error. The optical fibers 52 maybe prepared for ribbonization in a specific order that minimizes themaximum fiber core position error across the signal-fiber array 50.

FIG. 41 illustrates iterations of an algorithm that randomly swaps two(2) optical fibers 52 in the signal-fiber array 50, assesses the maximumfiber core position error, and logs preferred orderings of the opticalfibers 52 that yield low values for maximum fiber core position error.These preferred orderings of optical fibers 52 may be used to seedsubsequent algorithm runs, allowing the algorithm to converge on optimalordering configuration of the optical fibers 52 of the signal-fiberarray 50.

Prior to coloring and ribbonization of the optical fibers 52,measurements of diameter of the cladding 74 of the optical fibers 52 isperformed on a set of N fibers that will eventually be fabricated into aribbonized multifiber cable 60 and then a signal-fiber array 50 of anFAU 10. In one variant of the algorithm, the N fibers are treated as aclosed group or set. Prior to making a decision on fiber ribbon positionand corresponding fiber color, the algorithm is executed to determinethe optimal fiber ordering in the fiber ribbon.

The algorithm starts with a random ordering of optical fibers, such asthe 8-fiber array showing in iteration A of FIG. 41 . The algorithmdetermines the position of the cores 72 of the optical fibers 52 of thesignal-fiber array 50 (assuming no FCE) and then estimates the maximumfiber core total error for the configuration.

Next at iteration B, two of the optical fibers 52 are selected at randomand their positions are swapped in the signal-fiber array 50. Theresulting configuration may then be assessed for maximum fiber coretotal error. The random swapping of fibers is continued throughiteration C for a predetermined number (P) of swaps (for example P=100).The results for maximum fiber core position error for the P swapconfigurations may then be evaluated. The configuration of opticalfibers 52 with the lowest maximum fiber core position error may then beselected as the seed configuration for the next round of P swapconfigurations. The method continues until there are Q trials of P swapconfigurations, where, for example, Q=10. The configuration from Qtrials of P swap configurations with the best (e.g. lowest) overallmaximum fiber core total error may be selected for ribbonization. Thetotal physical width (W) of the signal-fiber array may be substantiallyunchanged by the various fiber swap configurations.

Additionally or alternatively, the method may determine an optimal orderof the optical fibers 52 of the signal-fiber array 50 by swappingbetween the set of N randomly selected optical fibers 52, and a set of Madditional optical fibers 52 with the similar diameter distribution ofthe cladding 74 that serve as a pool of available optical fibers for thesignal-fiber array 50. The method including swapping optical fibersbetween set N and additional set M of optical fibers 52 is illustratedin FIG. 42 . Swapping of optical fibers between set N and additional setM may reduce of maximum fiber core position error by about 3× ascompared to the ordering over the closed set of N optical fibers 52. Theswapping of optical fibers between set N and additional set M may enableadditional control over the width W of groups of N optical fibers, sothat the signal-fiber array 50 of N optical fibers 52 may have a width Wthat is extremely close to the ideal case (e.g., N*mean fiber claddingdiameter for an signal-fiber array 50 having N optical fibers 52).

Iteration A of FIG. 42 depicts an initial random configuration of theoptical fibers 52 of the signal-fiber array 50 and the set of Madditional optical fibers 52. Iteration B of FIG. 42 illustrates theresults of a swap of an optical fiber 52 of the signal-fiber array 50and the set of M additional optical fibers. The width W of thesignal-fiber array 50 is reduced, since a larger optical fiber 52 in thesignal-fiber array 50 was swapped with a smaller optical fiber 52 fromthe set of M optical fibers 52.

After each swap of a pair of optical fibers 52, the maximum fiber coreposition error is determined. Additionally, the total width of thesignal-fiber array 50 may also be determined. The algorithm may continueiterations, such as iteration C, to determine a configuration where thetotal width of the signal-fiber array 50 is within c of the target value(e.g., N*mean fiber cladding diameter) and the maximum fiber coreposition error is at a minimum. The algorithm may track of allconfiguration of the optical fibers 52 and store a configuration ofoptical fibers 52 that provides the lowest maximum fiber core positionerror. If a subsequent swap of the optical fibers 52 results in asignal-fiber array 50 with a width W within c of the target value andthe maximum fiber core position error is lower than previously storedvalue, the new configuration and corresponding properties are stored. Atthe completion of the iterations, the stored configuration of theoptical fibers 52 may have a width W of the signal-fiber array 50 thatsubstantially equal to the target value, and a comparatively low maximumfiber core position error.

The algorithm may also be applied to practical fiber ribbonizationsituations, such as where the optical fibers 52 in their spools havebeen colored. In this example, for a signal-fiber array having N opticalfibers 52, there are effectively N fiber pools (one pool for each fibercolor) and swapping between the signal-fiber array and a pool of opticalfibers, or set M additional optical fibers is limited to fibers of thesame color. The algorithm may be executed multiple times to generatemultiple configurations of signal-fiber arrays having N optical fibers52, with the set M optical fiber spools replenished with additional dataon colored fiber cladding diameter as additional spools of optical fiber52 become available.

In some example embodiments, an FAU 10 may be fabricated using aplurality of multifiber cables 60. The signal-fiber array 50 of the FAU10 may include interdigitated optical fibers 52 from the plurality ofmultifiber cables 60. FIG. 43 illustrates a data plot 124 for asignal-fiber array including thirty-two (32) optical fiber 52 fabricatedfrom two (2) multifiber cables 60 each including sixteen (16) opticalfibers 52. The data plot 124 indicates that the benefits of ordering theoptical fibers 52 to minimize the maximum fiber core position error forfabrication of signal-fiber arrays 50 with sixteen (16) optical fiber 52are preserved when interdigitating optical fibers 52 of two (2)multifiber cables 60 to create an FAU 10 including thirty-two (32)optical fibers 52.

In the data plot 124, a first set of sixteen (16) optical fibers 52 ischosen. The algorithm is then executed to determine an optical fiber 52ordering that minimizes the maximum fiber core position error. Theprocess is repeated for a second set of sixteen (16) randomly selectedoptical fibers 52, yielding two (2) sets of sixteen (16) optical fibers52 that each have a fiber ordering that minimizes the maximum fiber coretotal error.

In some example embodiments, the fiber cladding diameter variation andFCE variation along lengths of the optical fibers 52 on spools feedingthe fiber ribbonization process may also be considered. For example,using the two (2) sets of sixteen (16) optical fibers 52, a plurality ofsimulations, for example 1000 simulations, may be run imposingvariations in diameter of the cladding 74 of the optical fiber 52 alongthe length of the optical fiber. Fiber cladding diameter variationsimulations may address the practical problem of only knowing the startand end diameter of the cladding 74 of the optical fiber 52 on a spool.Additionally, simulations of random FCE of the optical fiber may beapplied along the length of the optical fiber 52. For each simulation,the two (2) sets of sixteen (16) optical fibers 52 are interdigitated toin an FAU 10 having a signal-fiber array 50 with thirty-two (32) opticalfibers 52.

For each of the 1000 configurations the two (2) sets of sixteen (16)optical fibers 52 for the signal-fiber array 50 having two (2) sets ofsixteen (16) optical fibers 52 the maximum fiber core total error iscalculated. The one thousand (1000) values of maximum fiber core totalerror may be sorted from lowest to highest to generate a data plot, suchas data plot 124. Data plot 124 shows ten (10) curves with similarperformance. The data plot 124 indicates that based on fiber claddingdiameter error accumulation, around 90% of all fabricated multifibercables, or fiber ribbons, have a maximum fiber core total error of lessthan about 0.52 μm.

In an example embodiment, an FAU may be fabricated using a side-by-sideconcatenation of order optimized optical fiber sets. For example, an FAU10 may be fabricated including seventy-two (72) optical fibers 52comprised of three (3) interdigitated groups of two (2) sets of twelve(12) optical fibers 52.

As shown in FIG. 44 in step A to step B, a first set of twelve (12)optical fibers 52 may be interdigitated with a second set of twelve (12)optical fibers 52 to for an interdigitated group of twenty-four (24)optical fibers 52. Both the first and second sets of twelve (12) opticalfibers 52 may have an been ordered to minimize maximum fiber core totalerror, as described above. At step C to step D, three (3) interdigitatedgroups of two (2) sets of twelve (12) optical fibers 52 may bepositioned on the support substrate 20. In an example embodiment, theoptical fibers 52 disposed at the adjacent edges of the interdigitatedgroups may be in direct contact with each other, such as at datumsurface 104. The lidless FAU squeeze process, as discussed above inreference to FIGS. 2-10B, may be used to fix the optical fibers 52 tothe support substrate 20. Using the 72-fiber interdigitated arrayapproach shown in FIG. 44 with optimal ordering of fibers in each of thesets of twelve (12) optical fibers 52 may reduce the maximum fiber coretotal error by, for example 15-40% compared to an FAU 10 includingseventy-two (72) optical fibers 52 selected randomly.

FIG. 45 illustrates and example embodiment, in which the signal-fiberarray 50 of the FAU 10 may be formed from a plurality of fiber arraygroups 58 of N optical fibers 52. The maximum fiber core total error maybe minimized by measuring the diameter of the cladding 74 of the opticalfibers 52 for each optical fiber 52 in each group of N optical fibers 52that form a fiber array group 58. When the fiber array groups 58 arearranged on the support substrate 20, the total width of thesignal-fiber array 50 may be estimated.

As larger signal-fiber arrays 50 are constructed by concatenating fiberarray groups 58, the accumulation of fiber core position error may beadjusted for by adding spacer fibers 59 between the fiber array groups58. The spacer fibers 59 may be fabricated with non-standard fibercladding diameters that may be either larger or smaller than the meanfiber cladding diameter. Based on the magnitude of the width correctionalong the signal-fiber array 50, a specific spacer fiber 59 may beselected and positioned between fiber array groups 58 to bring the cores72 of the optical fibers 52 of the fiber array groups 58 back intoalignment with the target pitch of the signal-fiber array 50.

FIG. 46 illustrates an apparatus for determining a maximum fiber coretotal error for a plurality of optical fibers in a plurality ofconfigurations according to an example embodiment. The apparatus of FIG.46 may be employed, for example, on a user device (e.g., a computer, anetwork access terminal, a personal digital assistant (PDA), cellularphone, smart phone, wearable computer, or the like) or a variety ofother devices (such as, for example, a network device, server, proxy, orthe like. Alternatively, embodiments may be employed on a combination ofdevices. Accordingly, some embodiments of the present disclosure may beembodied wholly at a single device or by devices in a client/serverrelationship. Furthermore, it should be noted that the devices orelements described below may not be mandatory and thus some may beomitted in certain embodiments.

In an example embodiment, the apparatus may include or otherwise be incommunication with processing circuitry 520 that is configured toperform data processing, application execution and other processing andmanagement services according to an example embodiment of the presentinvention. In one embodiment, the processing circuitry 520 may include amemory 524 and a processor 522 that may be in communication with orotherwise control a user interface 526 and a communication interface528. As such, the processing circuitry 520 may be embodied as a circuitchip (e.g., an integrated circuit chip) configured (e.g., with hardware,software or a combination of hardware and software) to performoperations described herein. However, in some embodiments, theprocessing circuitry 520 may be embodied as a portion of a server,computer, laptop, workstation or even one of various mobile computingdevices or wearable computing devices. In situations where theprocessing circuitry 520 is embodied as a server or at a remotelylocated computing device, the user interface 526 may be disposed atanother device (e.g., at a computer terminal or client device such as auser device that may be in communication with the processing circuitry520 via the communication interface 528 and/or a network.

The processor 522 may be embodied in a number of different ways. Forexample, the processor 522 may be embodied as various processing meanssuch as a microprocessor or other processing element, a coprocessor, acontroller or various other computing or processing devices includingintegrated circuits such as, for example, an ASIC (application specificintegrated circuit), an FPGA (field programmable gate array), a hardwareaccelerator, or the like. In an example embodiment, the processor 522may be configured to execute instructions stored in the memory 524 orotherwise accessible to the processor 522. As such, whether configuredby hardware or software methods, or by a combination thereof, theprocessor 522 may represent an entity (e.g., physically embodied incircuitry) capable of performing operations according to embodiments ofthe present invention while configured accordingly. Thus, for example,when the processor 522 is embodied as an ASIC, FPGA or the like, theprocessor 522 may be specifically configured hardware for conducting theoperations described herein. Alternatively, as another example, when theprocessor 522 is embodied as an executor of software instructions, theinstructions may specifically configure the processor 522 to perform theoperations described herein.

In an example embodiment, the memory 524 may include one or morenon-transitory storage or memory devices such as, for example, volatileand/or non-volatile memory that may be either fixed or removable. Thememory 524 may be configured to store information, data, applications,instructions or the like for enabling the apparatus to carry out variousfunctions in accordance with example embodiments of the presentinvention. For example, the memory 524 could be configured to bufferinput data for processing by the processor 522. Additionally oralternatively, the memory 524 could be configured to store instructionsfor execution by the processor 522. As yet another alternative, thememory 524 may include one of a plurality of databases that may store avariety of files, contents or data sets. Among the contents of thememory 524, applications (e.g., client applications or serviceapplication) may be stored for execution by the processor 522 in orderto carry out the functionality associated with each respectiveapplication.

The user interface 526 may be an input/output device for receivinginstructions directly from a user. The user interface 526 may be incommunication with the processing circuitry 520 to receive user inputvia the user interface 526 and/or to present output to a user as, forexample, audible, visual, mechanical or other output indications. Theuser interface 526 may include, for example, a keyboard, a mouse, ajoystick, a display (e.g., a touch screen display), a microphone, aspeaker, or other input/output mechanisms. Further, the processingcircuitry 520 may comprise, or be in communication with, user interfacecircuitry configured to control at least some functions of one or moreelements of the user interface 526. The processing circuitry 520 and/oruser interface circuitry may be configured to control one or morefunctions of one or more elements of the user interface 526 throughcomputer program instructions (e.g., software and/or firmware) stored ona memory device accessible to the processing circuitry 520 (e.g.,volatile memory, non-volatile memory, and/or the like). In some exampleembodiments, the user interface circuitry is configured to facilitateuser control of at least some functions of the apparatus through the useof a display configured to respond to user inputs. The processingcircuitry 520 may also comprise, or be in communication with, displaycircuitry configured to display at least a portion of a user interface526, the display and the display circuitry configured to facilitate usercontrol of at least some functions of the apparatus.

The communication interface 528 may be any means such as a device orcircuitry embodied in either hardware, software, or a combination ofhardware and software that is configured to receive and/or transmit datafrom/to a network and/or any other device or module in communicationwith the apparatus. The communication interface 528 may also include,for example, an antenna (or multiple antennas) and supporting hardwareand/or software for enabling communications with the network or otherdevices (e.g., a user device). In some environments, the communicationinterface 528 may alternatively or additionally support wiredcommunication. As such, for example, the communication interface 528 mayinclude a communication modem and/or other hardware/software forsupporting communication via cable, digital subscriber line (DSL),universal serial bus (USB) or other mechanisms. In an exemplaryembodiment, the communication interface 528 may support communicationvia one or more different communication protocols or methods. In somecases, IEEE 802.15.4 based communication techniques such as ZigBee orother low power, short range communication protocols, such as aproprietary technique based on IEEE 802.15.4 may be employed along withradio frequency identification (RFID) or other short range communicationtechniques. In other embodiments, communication protocols based on thedraft IEEE 802.15.4a standard may be established.

Adhesive Profile Control and Passive Alignment

Controlling the shape of adhesive over optical fibers may be animportant parameter in lidless FAU fabrication that is not a factor inlidded FAUs, since the optical fibers are generally completely coveredby adhesive and a glass lid. Lidless FAUs may be utilized in compact,low-profile, interconnections, where a lid would interfere with otheroptical or electrical components, or otherwise increase the volume ofthe optical interconnection solution. Therefore, it may be advantageousfor the adhesive over the fibers to be sufficiently thin to enable theseinterconnections.

An additional value of controlling the adhesive layer profile is thatexcessive adhesive may lead to shrinkage and shifts in optical fiberposition during adhesive curing and environmental testing. In instancesin which the adhesive is not deposited symmetrically over the fiberarray, the differences in adhesive shrinkage (e.g., on the left vs.right side of the optical fiber) may cause fiber shifts in curing andenvironmental testing. These shifting effects may be reduced by reducingthe thickness of the adhesive.

A further benefit of controlling and reducing the adhesive thicknesslayer may be accelerated curing of the adhesive. This may beparticularly advantageous for FAUs assembled using inorganic adhesives,such as sodium silicate (“liquid glass”). Sodium silicate adhesives mayexperience cracking if cured too quickly or deposited in an excessivelythick layer. In some example embodiments, the height of the adhesive maybe less than half of the diameter of an optical fiber of signal-fiberarray.

The solution described below enables the adhesive layer profile to beprecisely controlled, and in locations between optical fibers where itis not needed, it may be completely eliminated. This approach may assistin mechanically decoupling individual fibers from each other, so thatthey are not joined together with a thick layer of adhesive with adifferent CTE (Coefficient of Thermal Expansion) than the optical fiberor the base glass substrate.

Further, various techniques for molding the adhesive into a desiredshape, and simultaneously positioning fibers in a precise location in anarray on a V-groove substrate or a flat glass sheet are also described.For example, the adhesive around the optical fibers may be molded with aprecise profile to enable passive alignment of the optical fibers toV-groove alignment features or PIC alignment features.

Lidless FAUs 11 may be assembled using either a permanent V-groovesubstrate 21 (that remains as part of the FAU after assembly), or atemporary V-groove substrate, for aligning signal-fiber arrays 50. FIGS.47 and 48 depict an assembly process of a lidless FAU 11 using apermanent V-groove substrate 21. A multifiber ribbon 60 may be strippedand individual bare optical fibers 52 of the signal-fiber array 50 maybe aligned with V-grooves 700 in the V-groove substrate 21. A releasepad 204 pad mounted on a top plate 202 may be positioned over theV-groove substrate 21. The release pad 204 may be constructed of amaterial with high release properties from the chosen adhesive, such asSilicone or Buna-N. Alternatively, a thin release sheet or film can beapplied to the release pad 204 to improve the release properties, suchas a PTFE sheet. UV curable adhesive 70 may be applied over the bareoptical fibers 52 in the V-grooves 700 in the V-groove substrate 21.Additionally or alternatively, the UV curable adhesive 70 may be appliedover the V-groove substrate 21 before the optical fibers 52 are insertedinto the V-grooves 700. The release pad 204 may be lowered onto thesignal-fiber array 50 and downward force is applied through the topplate 202, causing the release pad 204 to deform around each opticalfiber 52 in the signal-fiber array 50, as depicted in FIG. 49 . Theadhesive 70, applied over the optical fibers 52 may be displaced by thedeformation of the release pad 204, resulting in a thin layer ofadhesive 70 disposed between the optical fibers 52 and the V-groovesubstrate 21. In the embodiment shown, the lidless FAU 11 does notrequire a lid, such as a glass lid or other substrate, disposed on thetop of the optical fibers 52 opposite the V-groove substrate 21. Thisenables each of the plurality of optical fibers 52 to define a firstexposed datum surface 103 at a top of the optical fibers 52 opposite theV-groove substrate 21, as depicted in FIG. 51 .

In an alternative assembly approach, the release pad 204 may be appliedover the signal-fiber array 50 with a reduced force, which may not causefull deformation of the release pad 204 around each optical fiber 52. Inthis process, there may be a small vertical gap 702 between a bottomsurface of the release pad 204 and the top surface of the V-groovesubstrate 21 in regions between each optical fiber 52 in thesignal-fiber array 50, as depicted in FIG. 50 . This vertical gap 702forms a capillary channel that enables adhesive to wick into the regionbetween the release pad 204, the V-groove substrate 21, and neighboringoptical fibers 52 in the signal-fiber array 50. As adhesive 70 isapplied to either end of the V-groove substrate 21, but particularly atthe fiber array end face, adhesive 70 may also wick into the smallV-shaped channel 704 formed below each optical fiber 52 in thesignal-fiber array 50.

The process of adhesive wicking may limit or prevent entrapped bubblesin the adhesive 70, which may be more common when adhesive 70 is appliedover the signal-fiber array 50 prior to the lowering of the release pad204 onto the signal-fiber array 50. The progress of adhesive 70 as itwicks into the vertical gap 702 and/or the V-shaped channel 704, may bemonitored during the adhesive application process by observing theV-groove substrate 21 from below, using, for example, a glass supportsubstrate with a camera and/or mirror reflection optics. Once theadhesive 70 has flowed to fill the vertical gap 702 and/or the V-shapedchannel 704 the downward force applied to the release pad 204 may beincreased to exclude it from regions around each optical fiber 52 in thesignal-fiber array 50.

The adhesive 70 may be cured by exposure to UV light, as discussed abovein reference to FIGS. 2-7B. The UV light may be transmitted toward theV-groove substrate 21 from the sides, either end, or even the bottom ofthe V-groove substrate 21, when using a UV-transparent glass supportsubstrate. After the adhesive 70 is UV-cured the release pad 204 may beremoved from the signal-fiber array 50, as depicted in FIG. 51 . Theheight of the adhesive 70 may be relatively thin, such as less thanfifty percent of the diameter of an optical fiber 52 of the ofsignal-fiber array 50, less than twenty-five percent of the diameter ofan optical fiber 52 of the of signal-fiber array 50, less than twentypercent of the diameter of an optical fiber 52 of the of signal-fiberarray 50, less than ten percent of the diameter of an optical fiber 52of the of signal-fiber array 50, or other suitable height.

An advantage of utilizing the release pad 204 to shape the adhesive 70over the signal-fiber array 50 of the lidless FAU 11 is that theadhesive profile may be easily modified by adjusting the downward forceapplied to the release pad 204. For example, if the downward force isrelatively low and/or the release pad 204 is significantly stiff, athick layer of adhesive may be formed in the region between opticalfibers, as depicted in FIG. 52A. If the downward force is increasedand/or the release pad 204 stiffness is reduced, the release pad 204 maypress downward more, thereby excluding more adhesive 70 from the regionbetween optical fibers 52 as depicted in FIGS. 52B and 52C. In eachcase, the downward force applied to the release pad 204 may besufficient to press the optical fibers 52 down into contact with theV-grooves 700 in the V-groove substrate 21. The disclosed process mayproduce an identical adhesive profile over each optical fiber 52 in thesignal-fiber array 50, because the downward force is substantiallyuniform over the signal-fiber array 50, and the release pad 504 has asubstantially uniform elasticity.

If the downward force applied to the release pad 204 is increased and/orthe release pad 204 is even softer, the release pad 204 may deform intothe small V-shaped regions 706 between the optical fiber 52 and the twoadjacent sidewalls of the V-groove 700, as depicted in FIG. 53A. If thepressing force applied to the release pad 204 is further increased, evenmore adhesive 70 may be excluded from the V-shaped regions 706, asdepicted in FIG. 53B. Exclusion of adhesive 70 from the V-shaped regions706 adjacent to each optical fiber 52 may be utilized to increase thebonding strength of a second adhesive layer applied over the lidless FAU11. For example, if a lid is subsequently bonded over the lidless FAU11, the V-shaped regions 706 provide angled sidewall bonding interfacesthat are partially in shear to resist delamination of the lid. TheV-shaped regions 706 may also increase the total bonding surface area ofany adhesive used for joining a top surface of the lidless FAU 11 toanother component.

In another example embodiment, lidless FAUs may be fabricated bypressing optical fibers onto a precision flat sheet of glass, e.g.support substrate 20, for example LCD glass, using a reusable V-groovealignment substrate 710. The assembly approach depicted in FIGS. 54 and55 , is similar to the assembly approach described above for lidlessFAUs 11, where signal-fiber array 50 is pushed into a V-groove substrate21. In this case, however, a reusable V-groove alignment substrate 700may be used to align optical fibers 52 of the signal-fiber array 50 onthe support substrate 20. V-grooves 712 in the V-groove alignmentsubstrate 710 may be coated with a non-stick coating, such as afluorosilane coating, a PTFE coating, or other suitable non-stickcoating. The V-grooves 712 may also be covered by a precision uniformthickness film sheet, such as a sheet of PTFE.

In FIG. 54 , the reusable V-groove alignment substrate 710 is positionedabove the signal-fiber array 50. An adhesive 70 may be pre-applied overthe signal-fiber array 50, or it may be allowed to wick into the gapbetween V-groove alignment substrate 710 and the support substrate 20after the V-groove alignment substrate 710 is lowered onto thesignal-fiber array 50.

Once the reusable V-groove alignment substrate 710 is lowered over thesignal-fiber array 50, a downward force may be applied to press theoptical fibers 52 into contact with the support substrate 20, whilespacing the optical fibers 52 in the V-grooves 712 on precise pitch, asdepicted in FIG. 55 . The adhesive 70 may flow into gap 714 between theV-groove alignment substrate 710 and the support substrate 20 and intothe V-grooves 712, as depicted in FIG. 56 .

The adhesive 70 may be cured via UV exposure while downward force isapplied on the V-groove alignment substrate 710. After curing of theadhesive 70, the V-groove alignment substrate 710 may be raised awayfrom the signal-fiber array 50. As shown in FIG. 57 , the adhesive 70around each of the optical fibers 52 of the signal-fiber array 50 maytake the shape of the associated V-groove 712 of the V-groove alignmentsubstrate 714, including raised peaks 730 over each optical fiber 52that correspond to the valleys in each V-groove 712. The height of theadhesive 70 disposed between adjacent optical fibers 52 may berelatively thin, such as less than fifty percent of the diameter of anoptical fiber 52 of the of signal-fiber array 50, less than twenty-fivepercent of the diameter of an optical fiber 52 of the of signal-fiberarray 50, less than twenty percent of the diameter of an optical fiber52 of the of signal-fiber array 50, less than ten percent of thediameter of an optical fiber 52 of the of signal-fiber array 50, orother suitable height.

Turning to FIG. 58 , a lid 719 may be optionally applied over thesignal-fiber array 50 of the lidless FAU 11. The lid may assiststabilization of the signal-fiber array 50. The lid 719 may rest on thepointed ridge peaks formed in the adhesive 70 by the V-grooves 712 ofthe V-groove alignment substrate 710. Additional adhesive 716 may bedisposed between the signal-fiber array 50 and the lid 716, e.g. betweenthe ridge peaks. The additional adhesive may bond the lid 719 to thesignal-fiber array 50. In FIG. 59 , excess adhesive 70 exists betweenthe lid 719 and the support substrate 20. Prior to attachment of the lid719, the pointed ridge peaks of the adhesive 70 over the optical fibers52 may be removed via, for example, a sanding operation that stops whenthe top 718 of the signal-fiber array 50 is reached, as indicated by thedotted line. After sanding is completed, the lid 719 may be applied overthe optical fibers 52 of the signal-fiber array 50, as shown in FIG. 60. The sanded adhesive 70 may present a more stable surface for seatingof the lid 719 and may provide a more compact profile. Additionally, thepointed ridge peaks of the adhesive profile or the substantially flatsanded surface of the adhesive 70 may define a datum surface 730. Thelid 719 may rest on the datum surfaces 730.

As an alternative to sanding the adhesive 70, the V-grooves 722 of analignment substrate 720 may be truncated, e.g. include flat bottoms, asdepicted in FIG. 61 . A bottom surface of the alignment substrate 720may be coated with a non-stick coating as described above. Adhesive 70may be applied over the optical fibers 52 in the signal-fiber array 50,and the V-groove alignment substrate 720 may lowered onto thesignal-fiber array 50, as depicted in FIG. 62 . As described above, theadhesive 70 may be cured while a downward force is applied on thealignment substrate 720. As shown in FIG. 63 , after UV exposure curingor the adhesive 70, the alignment substrate 720 may be lifted off thesignal-fiber array 50. The adhesive profile over the signal-fiber array50 may match the flat-bottom V-groove profile of the alignment substrate720. Thereby, eliminating the excess adhesive 70 over the signal-fiberarray 50 compared to the alignment substrate 720 include the fullV-grooves 712. The substantially flat surface of the profile of adhesive70 may define a datum surface 730. As shown in FIG. 64 , additionaladhesive 716 may be applied over the signal-fiber array, followed byapplication of a lid 719. The lid 719 may rest on the datum surfaces730.

In some example embodiments, the lid 719 may include a broad trench ornotch feature 724 disposed in a bottom surface, as shown in FIG. 65 .The notch feature 724 may be generated by sawing or grinding the lid721, as shown in FIG. 65 ). In an example embodiment, the depth andwidth dimensions may not need to be precisely controlled, as long as thenotch feature 724 enables clearance of the optical fibers 52 on thesupport substrate 20. This notch feature 724 may reduce the thickness ofadhesive 70, 716, disposed between the lid 719 and the support substrate20, thereby reducing the total depth of the FAU. Utilization of a notchfeature 724 particularly effective when the optical fibers 52 arearranged on fine pitch (e.g., 127 μm pitch for 125 μm diameter fibers),which may reduce or eliminate the amount of adhesive 70 between adjacentoptical fibers 52.

The profile, e.g. shape, of the adhesive 70 over the signal-fiber array50 of the lidless FAU 11 may be utilized to passively align the lidlessFAU 11 to other optical components. In the example depicted in FIGS. 66Aand 66B, the lidless FAU 11 is positioned over an array of V-grooves 802on a PIC substrate 800. The fabrication process of the lidless FAU 11positions the V-grooves 712 of the V-groove alignment substrate 710 onprecise pitch on the support substrate 20, causing the optical fibers 52and profile of the adhesive 70 to be precisely aligned to the V-grooves802 on the PIC substrate 800.

The PIC substrate 800 may include waveguides 804 that are aligned to thecenters of the V-grooves 802 and may be disposed at a precisepredetermined depth. The depth of the waveguides 804 may enable precisealignment to optical fiber cores 72 to the waveguides 804. The lidlessFAU 11 may be inserted into the V-grooves 802 of the PIC substrate 800.The profiled shape of the adhesive 70 may guide the optical fibers 52into contact with the sidewalls of the V-groove 802. Adhesive 806 may beapplied in a gap between the PIC substrate 800 and the lidless FAU 11.The application of the adhesive 806 may generate a large area thin bondline that provides a robust mechanical bond between the lidless FAU 11and the PIC substrate 800.

As discussed above in reference to FIGS. 15-21 , a lidless FAU 11 may bepassively aligned to a notch feature 302, in the waveguide substrate 300or PIC substrate 400, for interconnection of the optical fibers 52 towaveguides 304. The approach may be enabled by an absence of adhesive 70sidewalls of the two outboard optical fibers 52 in the signal-fiberarray 50, defining datum surfaces 104 disposed on each edge opticalfiber 52, e.g. optical fibers 52 that have only one adjacent opticalfiber 52. The exposed datum surfaces 104 may enable lateral alignment ofthe lidless FAU 11 with the waveguide substrate 300 or PIC substrate400, such as mating with a precision surface disposed on the waveguidesubstrate 300 or PIC substrate 400.

A similar approach may be utilized with the lidless FAU fabricationapproach as described in reference to FIGS. 67-69 . FIG. 67 shows analignment substrate 720 with a bottom surface that has been given aprecision profile including a plurality of alignment grooves 723 by, forexample, a diamond turning process. The bottom surface of the alignmentsubstrate 720 may be coated with a non-stick coating, and pressed overplurality of optical fibers 52 of the signal fiber array 50 that havebeen previously covered with adhesive 70.

Force may be applied on the top surface of the alignment substrate 720to press the optical fibers 52 in the signal-fiber array 50 downwardinto contact with the surface of the support substrate 20. The adhesive70 may be UV cured and the alignment substrate 720 may be lifted off thesignal-fiber array. The cured adhesive 70 may form a thin layer (e.g.,<0.2-0.3 μm) over each optical fiber, with a surface profile thatmatches the profile of the bottom surface of the alignment substrate720. The molding process of the adhesive 70 precisely positions theoptical fibers 52 on the support substrate 20 (at a precision pitch),and also ensures that only an extremely thin layer of adhesive 70remains over the sides of the optical fibers 52. The resulting geometryof the lidless FAU 11 that is similar to the one used in lidless“squeeze” FAUs discussed above, but where the optical fibers 52 can bespaced apart and positioned relative to each other with arbitraryspacings.

Turning to FIG. 69 , the lidless FAU 11 may be coarsely aligned to analignment trench 402, or notch feature, disposed in a surface of the PICsubstrate 400. The PIC substrate 400 also includes a plurality ofwaveguides 404 that are positioned to align with the cores of theoptical fibers 52 of the lidless FAU 11 when the signal-fiber array 50is inserted into the PIC alignment trench.

FIG. 70 provides an end face view of the lidless FAU 11 as it iscoarsely aligned above the alignment trench 402 of the PIC substrate400. The adhesive profile of the above process results in no adhesive 70disposed over the sides of the two outboard fibers in the array,allowing these surfaces to be used as precision alignment datumssurfaces 104 during assembly.

In FIG. 71 , the lidless FAU 11 has been inserted into the alignmenttrench 402 of the PIC substrate 400. The depth of the alignment trench402 is precisely controlled during the fabrication process, such thatthe cores of the optical fiber 52 are vertically aligned with thewaveguides 404 of the PIC substrate 400. The cores of the optical fibers52 of the lidless FAU 10 are horizontally aligned to the waveguides 404by alignment of outboard fiber side surfaces, e.g. datum surfaces 104,with the sidewalls of the alignment trench 402. In some embodiments, thealignment trench 402 may be slightly wider than the signal-fiber array50, by, for example, 0.5 μm, such that the signal-fiber array 50 may beinserted into the alignment trench 402 without mechanical interference.

Adhesive 810 may be applied to the interface after the lidless FAU 11has been aligned to the alignment trench 402 of the PIC substrate, byallowing adhesive 810 to wick into the gap region between the lidlessFAU 11 and the alignment trench 402. Alternatively, adhesive 810 may beapplied to the alignment trench 402 and/or the optical fibers 52 of thelidless FAU 11 prior to assembly.

FIGS. 72 and 73 provide an example where two adhesive spacer regions 812of thickness or height (h) are formed on both sides of the signal-fiberarray 50 of the lidless FAU 11. The height (h) of the spacer region isselected to equal (r−d), where r is the radius of the optical fiber 52,and d is the depth of the center of PIC waveguide 404 below the topsurface of PIC substrate 400. The molding process for shaping theprofile of the adhesive 70 over the signal-fiber array 50 of the lidlessFAU 11 may be utilized to form alignment features 235 at a precisionoffset relative to the optical fibers. For example, in the exampleembodiment discussed above in reference to FIGS. 70 and 71 , the lidlessFAU 11 was vertically aligned in the alignment trench 402 because thealignment trench 402 had been formed to be a precise depth. In theembodiment depicted in FIGS. 72 and 73 , the vertical alignment of thecores of the optical fibers 52 of the lidless FAU 11 to the waveguides404 be determined by the thickness of two spacer regions 812 formed fromthe cured adhesive 70 on each side of the signal-fiber array 50. Usingthis approach, the alignment trench 402 may be made deeper, such thatthe alignment trench 402 does not contact the signal-fiber array 50during insertion of the lidless FAU into the PIC alignment trench.

The height (h) of the spacer region may be established by the precisionshape of the diamond turned alignment substrate 720 that defines theprofile of the adhesive 70. The depth of the adhesive shaping surfacemay be adjusted to take into account the shrinkage of UV curableadhesive 70 during curing. Inorganic fillers in the adhesive 70 may alsobe used to reduce shrinkage during curing of the adhesive 70. The heightof the adhesive 70 in the spacer regions 812 may be relatively thin,such as less than fifty percent of the diameter of an optical fiber 52of the of signal-fiber array 50, less than twenty-five percent of thediameter of an optical fiber 52 of the of signal-fiber array 50, lessthan twenty percent of the diameter of an optical fiber 52 of the ofsignal-fiber array 50, less than ten percent of the diameter of anoptical fiber 52 of the of signal-fiber array 50, or other suitableheight.

During assembly, the lidless FAU 11 may be inserted into the alignmenttrench 402 until the spacer region 812 contacts the top surface of thePIC substrate 400. As described above, the lidless FAU 11 may then bejoined to the PIC substrate 400 by UV curing adhesive 70 that disposedin the gap between the lidless FAU 11 and the PIC substrate 400.

In the depicted embodiment, the spacer region 812 is formed as a largeflat region adjacent to the outboard optical fibers 52. The adhesive 70that joins the lidless FAU 11 to the PIC substrate 400 may be free toflow into the region around the spacer region 812, ensuring that a verythin layer of joining adhesive remains between the spacer region 812 andthe top of the PIC substrate 400, for example the joining adhesive maybe less than or equal to 0.5 μm thick.

In the example depicted in FIGS. 74-76 , a precision alignment feature835, such as a trench or raised region, may be formed into the profileof the adhesive 70. The alignment feature 835 may be formed at a precisehorizontal distance from the signal-fiber array 50. As depicted in FIGS.74 and 75 , the alignment substrate 720 includes mold features 733configured for forming alignment features 835 horizontally offset fromthe V-grooves 722 configured to align the optical fibers 52.

The alignment substrate 720 may be pressed down over the adhesive and asignal-fiber array 50 disposed on the support substrate 20. Once theadhesive 70 cures, the alignment feature 835 is formed at a precisionhorizontal and vertical offset from the optical fibers 52.

The alignment substrate 720 may be removed from the adhesive a 70 andsupport substrate 20 aided by the non-stick surface coating. Once thealignment substrate is removed, the alignment feature 835 may enablepassive alignment of the support substrate 20 and the signal-fiber array50 to waveguides 404 on the PIC substrate 400 or other substrate. Forexample, if the PIC substrate 400 includes a precision etched notchfeature or alignment trench 402, the alignment feature 835 may be sizedand positioned to passively align with complementary features disposedon the PIC substrate 400 to enable precision alignment of cores of theoptical fibers 52 to waveguides 404 of the PIC substrate 400.

The alignment feature 835 may be formed in any shape useful for aligningthe cores of the optical fibers 52 with precision features on anothersubstrate or component, including raised features, such as posts,rectangles, triangles, ridges, or the like or depressed features, suchas trenches and pits or other depressions with any perimeter profile.

In an example embodiment, a fiber optic assembly is provided including asupport substrate having a substantially flat surface and a signal-fiberarray supported on the support substrate. The signal-fiber arrayincludes a plurality of optical fibers. At least some of the opticalfiber of the plurality of optical fibers includes a first datum contactdisposed between the optical fiber and an adjacent optical fiber andeach of the optical fibers of the plurality of optical fibers includes asecond datum contact disposed between each of the optical fibers of theplurality of optical fibers and the support substrate. A first datumsurface is disposed at a top surface of each of the plurality of opticalfibers opposite the support surface.

In an example embodiment, the fiber optic assembly does not comprise alid disposed opposite the support substrate. In some example embodimentsthe fiber optic assembly also includes an adhesive disposed in one ormore voids disposed between the plurality of optical fibers or theplurality of optical fibers and the support substrate, the adhesive isnot disposed at the first or second datum contact. In an exampleembodiment, a second datum surface is disposed on each edge opticalfiber of the plurality of optical fibers and the second datum surface isdisposed opposite the first datum contact. In some example embodiments,the adhesive is not disposed in an area of the planar surface adjacentto the plurality of optical fibers. In an example embodiment, theplurality of optical fibers includes a plurality of D-shaped opticalfibers. In some example embodiments, a flat portion of each of theplurality of D-shaped optical fibers is disposed opposite to the supportsubstrate. In an example embodiment, a flat portion of each of theplurality of D-shaped optical fibers is disposed in contact with thesupport substrate. In some example embodiments, each of the plurality ofoptical fibers includes a coated portion and a bare glass portion andthe bare glass portion is disposed in the planar surface. In an exampleembodiment, the bare glass portion extends beyond an edge of the supportsubstrate. In some example embodiments, the adhesive is disposed on thebare glass portion between the support substrate and the coated portion,such that the adhesive provides strain relief. In an example embodiment,the planar surface comprises a step down surface. The bare glass portionis disposed on the planar surface and the coated portion is disposed onthe step down surface. In some example embodiments, the adhesive isdisposed on support substrate at the interface of the bare glass portionand the coated portion, such that the adhesive provides strain relief.In an example embodiment, the fiber optic assembly also includes awaveguide substrate including a plurality of waveguides and a notchconfigured to receive the plurality of optical fibers therein. A seconddatum surface is disposed on each edge optical fiber of the plurality ofoptical fibers and the second datum surface is disposed opposite thefirst datum contact. The first datum surface or the second datum surfacedefines the interface of the plurality of optical fibers and the notchenabling passive alignment of the plurality of optical fibers to theplurality of waveguides. In some example embodiments, the first datumsurface and the second datum surface define the interface of theplurality of optical fibers and the notch. In an example embodiment, thenotch has at least one precision surface. The interface of the pluralityof optical fibers and the notch is defined by the first datum surface orthe second datum surface and the at least one precision surface. In someexample embodiments, the fiber optic assembly also includes a waveguidesubstrate including a plurality of waveguides and an alignment featuredisposed a predetermined offset distance from the plurality ofwaveguides. The support substrate includes an alignment edge andinteraction of the alignment feature with the alignment edge enablespassive alignment of the plurality of optical fibers to the plurality ofwaveguides. In an example embodiment, the alignment edge of the supportsubstrate includes a precision surface.

In another example embodiment, a fiber optic assembly is providedincluding a fiber array unit including a support substrate having aplanar surface and a signal-fiber array supported on the first surfaceof the support substrate. The signal-fiber array includes a plurality ofoptical fibers and an adhesive disposed in one or more voids disposedbetween the plurality of optical fibers or the plurality of opticalfibers and the support substrate. Each optical fiber of the plurality ofoptical fibers includes a first datum contact disposed between theoptical fiber and an adjacent optical fiber and each of the opticalfibers of the plurality of optical fibers a second datum contactdisposed between each of the optical fibers of the plurality of opticalfibers and the support substrate. A first datum surface is disposed at atop surface of each of the plurality of optical fibers opposite thesupport surface. The fiber optic assembly also includes a waveguidesubstrate including a plurality of waveguides and a notch configured toreceive the plurality of optical fibers therein. A first datum surfaceis disposed at a top surface of each of the plurality of optical fibersopposite the support substrate and a second datum surface is disposed oneach edge optical fiber of the plurality of optical fibers, wherein thesecond datum surface is disposed opposite the first datum contact. Thefirst datum surface or the second datum surface defines the interface ofthe plurality of optical fibers and the notch enabling passive alignmentof the plurality of optical fibers to the plurality of waveguides. In anexample embodiment, the fiber array does not comprise a lid disposedopposite the support substrate.

In a further example embodiment, a method for fabricating a multifiberassembly is provided including selecting a plurality of optical fibersthat each have a respective cladding diameter and determining a maximumfiber core position error for the plurality of optical fibers in aplurality of configurations. For each configuration, the plurality ofoptical fibers are arranged side-by-side in an array such that eachoptical fiber has a position in the array, the plurality of opticalfibers are arranged in a different order, each optical fiber has arespective fiber core position relative to an ideal core position forthat optical fiber to define a respective fiber core position error, theideal core positions are based on each optical fiber of the plurality ofoptical fibers having an ideal cladding diameter, and the maximum fibercore position error is a maximum of the respective fiber core positionerrors. The method also includes determining a desired order of theplurality of optical fibers that minimizes the maximum fiber coreposition error.

In an example embodiment, for each optical fiber of the plurality ofoptical fibers, a geometric center of the respective cladding diameteris used as the respective fiber core position. In some exampleembodiment, the method also includes measuring the respective claddingdiameter of each optical fiber of the plurality of optical fibers at oneor more locations along a length of the optical fiber. In an exampleembodiment, the method also includes arranging the plurality of opticalfibers in the desired order and applying a matrix material to theplurality of optical fibers to form an optical fiber ribbon. In someexample embodiment, the determining the maximum fiber core positionerror includes arranging the plurality of optical fibers in a firstconfiguration, determining a first maximum fiber core position error forthe first configuration, swapping the positions of two optical fibers ofthe plurality of optical fibers, such that the plurality of opticalfibers are in a second configuration, and determining a second maximumfiber core position error for the second configuration. In an exampleembodiment, the swapping positions of two optical fibers of theplurality of optical fibers and the determining the maximum fiber coreposition error for a given configuration of the plurality ofconfigurations is repeated for a predetermined number of iterations. Insome example embodiment, the predetermined number of iterations includesall possible configurations of the plurality of optical fibers. In anexample embodiment, the predetermined number of iterations includes atleast 100 iterations. In some example embodiment, the method alsoincludes selecting a plurality of additional optical fibers anddetermining the maximum fiber core position error includes arranging theplurality of optical fibers in a first configuration, determining afirst maximum fiber core position error for the first configuration,swapping positions of two optical fibers of the plurality of opticalfibers and swapping a first optical fiber of the plurality of opticalfibers with a second optical fiber of the plurality of additionaloptical fibers to form a second configuration, and determining a secondmaximum fiber core position error for the second configuration. In anexample embodiment, each optical fiber of the plurality of opticalfibers has is colored and each of the additional optical fibers iscolored. The swapping the first optical fiber of the plurality ofoptical fibers with the second optical fiber of the plurality ofadditional optical fibers comprises swapping the first optical fiberwith a second optical fiber of the same color. In some exampleembodiment, the swapping positions of two optical fibers and thedetermining of a maximum fiber core position error is repeated for apredetermined number of iterations. In an example embodiment, thepredetermined number of iterations includes all possible configurationsof the plurality of optical fibers and the plurality of additionaloptical fibers. In some example embodiment, the predetermined number ofiterations includes at least 100 iterations.

In yet another example embodiment, a method for fabricating a fiberarray unit is provided including selecting a plurality of optical fibersthat each have a respective cladding diameter and determining a maximumfiber core position error for the plurality of optical fibers in aplurality of configurations. For each configuration, the plurality ofoptical fibers are arranged side-by-side in an array such that eachoptical fiber has a position in the array, the plurality of opticalfibers are arranged in a different order, each optical fiber has arespective fiber core position relative to an ideal core position forthat optical fiber to define a respective fiber core position error, theideal core positions are based on each optical fiber of the plurality ofoptical fibers having an ideal cladding diameter, and the maximum fibercore position error is a maximum of the respective fiber core positionerrors. The method also includes determining a desired order of theplurality of optical fibers that minimizes the maximum fiber coreposition error, positioning the plurality of optical fibers on a supportsubstrate in the configuration that includes the desired order, andapplying an adhesive to affix the plurality of optical fibers to thesupport substrate.

In some example embodiments, for each optical fiber of the plurality ofoptical fibers, a geometric center of the respective cladding diameteris used as the respective fiber core position. In an example embodiment,the method also includes measuring the respective cladding diameter ofeach optical fiber of the plurality of optical fibers at one or morelocations along a length of the optical fiber. The method also includes,the method also includes arranging the plurality of optical fibers inthe desired order, applying a matrix material to the plurality ofoptical fibers to form an optical fiber ribbon, removing the matrixmaterial from a portion of the plurality of optical fibers positioned onthe support substrate. In an example embodiment, the determining themaximum fiber core position error includes arranging the plurality ofoptical fibers in a first configuration, determining a first maximumfiber core position error for the first configuration, swapping thepositions of two optical fibers of the plurality of optical fibers, suchthat the plurality of optical fibers are in a second configuration, anddetermining a second maximum fiber core position error for the secondconfiguration. In some example embodiments, the swapping positions oftwo optical fibers of the plurality of optical fibers and thedetermining of the maximum fiber core position error for a givenconfiguration of the plurality of configurations is repeated for apredetermined number of iterations. In an example embodiment, the methodalso includes selecting a plurality of additional optical fibers and thedetermining the maximum fiber core total error includes arranging theplurality of optical fibers in a first configuration, determining afirst maximum fiber core position error for the first configuration,swapping positions of two optical fibers of the plurality of opticalfibers or swapping a first optical fiber of the plurality of opticalfibers with a second optical fiber of the plurality of additionaloptical fibers to form a second configuration, and determining a secondmaximum fiber core position error for the second configuration. In someexample embodiments, the swapping positions of two optical fibers andthe determining of a current maximum fiber core total error for acurrent configuration is repeated for a predetermined number ofiterations. In an example embodiment, the plurality of optical fibersincludes a first plurality of optical fibers and the desired orderincludes a first desired order. The method also includes selecting asecond plurality of optical fibers that each have a respective claddingdiameter, determining a plurality of maximum fiber core position errorsfor the second plurality of optical fibers in a second plurality ofconfigurations, determining a second determined order of the secondplurality of optical fibers that minimizes the maximum fiber core totalerror, positioning the second plurality of optical fibers in the seconddetermined order, interdigitating the first plurality of optical fibersin the first desired order and the second plurality of optical fibers inthe second desired order to form a interdigitated group of opticalfibers, positioning the interdigitated group of optical fibers on thesupport substrate, and applying the adhesive to affix the interdigitatedgroup of optical fibers to the support substrate. In some exampleembodiments, the interdigitated group of optical fibers includes a firstinterdigitated group of optical fibers and the method also includespositioning a second interdigitated group of optical fibers on thesupport substrate and applying the adhesive to affix the secondinterdigitated group of optical fibers to the support substrate. In anexample embodiment, the second interdigitated group of optical fibers isin direct contact with the first interdigitated group of optical fibers.In some example embodiments, the second interdigitated group of opticalfibers is spaced apart from the first interdigitated group of opticalfibers and the method also includes positioning a spacer fiber on thesupport substrate between the first interdigitated group of opticalfibers and the second interdigitated group of optical fibers andapplying the adhesive to affix the spacer fiber to the supportsubstrate.

In a further example embodiment, a optoelectronic assembly is providedincluding a photonic integrated circuit (PIC) including at least oneelectronic connection element and plurality of waveguides disposed on aPIC face, a printed circuit board (PCB) including at least one PCBelectronic connection element, which is complementary to the at leastone electronic connection element of the PIC and the PIC is configuredto be flip chip mounted to the PCB, a lidless fiber array unit includinga support substrate having a substantially flat first surface and asignal-fiber array including a plurality of optical fibers supported onthe first surface, and an alignment substrate disposed on the PIC faceand configured to align the plurality of optical fibers of thesignal-fiber array with the plurality of waveguides.

In an example embodiment, the alignment substrate comprises a precisionchannel configured to receive the lidless fiber array unit. In someexample embodiments, the optoelectronic assembly also includes analignment feature extending from the PIC face and configured to alignthe precision channel with the plurality of waveguides. In an exampleembodiment, the alignment feature includes an alignment rib including astop extending perpendicularly from a longitudinal axis of the alignmentrib. In some example embodiments, the PCB includes a recess and at leasta portion of the alignment substrate is disposed in the recess. In anexample embodiment, the optoelectronic assembly also includes an overlapsheet disposed on a second surface of the support substrate, oppositethe first surface. A portion of the overlap sheet extends past a forwardedge of the lidless fiber array, such that the portion of overlap sheetcovers at least a portion of the PIC when the lidless fiber array isinstalled in the alignment substrate. In some example embodiments, anadhesive is disposed between the portion of the overlap sheet and theportion of the PIC. In an example embodiment, the photoelectric assemblyalso includes a cap disposed on the PIC, the cap is configured to retainthe lidless fiber array unit in contact with the plurality ofwaveguides. In some example embodiments, the cap includes one of aprotrusion and a depression and the other of the protrusion and thedepression is disposed on the support substrate, such that theengagement of the depression by the protrusion resists removal of thelidless fiber optic array from the cap. In an example embodiment, thealignment substrate includes a precision channel configured to receivethe lidless fiber array unit and the optoelectronic assembly alsoincludes a clip feature disposed on the PIC. The clip feature includinga first set of arms extending away from an edge of the PIC andconfigured to engage side edges of the support substrate and a gripelement configured to bias the support substrate toward the precisionchannel. In some example embodiments, each of the side edges of thesupport substrate include a respective notch and each arm of the firstset of arms includes a catch configured to engage the respective notch.In an example embodiment, the clip feature includes a metal clip. Insome example embodiments, the clip feature also includes a second set ofarms extending across a portion of the PIC face and configured to engagethe alignment substrate. In an example embodiment, each arm of thesecond set of arms includes a catch configured to engage a sidewall ofthe alignment substrate.

In still further example embodiments, a fiber optic assembly is providedincluding a support substrate having a first surface comprising aplurality of V-grooves and a signal-fiber array supported on the firstsurface of the support substrate. The signal-fiber array including aplurality of optical fibers disposed in the plurality of V-grooves. Thefiber optic assembly also including an adhesive disposed on theplurality of optical fibers and the support substrate. A first datumsurface is disposed at a top of each of the plurality of optical fibersopposite the support surface.

In an example embodiment, the fiber optic assembly does not include alid disposed opposite the support substrate. In some exampleembodiments, the adhesive is disposed on the first surface between twooptical fibers of the plurality of optical fibers. In some exampleembodiments, the height of the adhesive is less than half of thediameter of an optical fiber of the plurality of optical fibers. In anexample embodiment, the adhesive is disposed in the V-grooves andexcluded from the first surface between two optical fibers of theplurality of optical fibers.

In another example embodiment, a fiber optic assembly is providedincluding a support substrate having a substantially planar surface anda signal-fiber array supported on the planar surface of the supportsubstrate. The signal-fiber array includes a plurality of opticalfibers. The fiber optic assembly also includes an adhesive disposed onthe plurality of optical fibers and the support substrate. Each of theoptical fibers is spaced from adjacent optical fibers of the pluralityof optical fibers at a precise pitch.

In an example embodiment, the adhesive includes an adhesive profilehaving a plurality of raised peaks over at least a portion of eachoptical fiber of the plurality of optical fibers. In some exampleembodiments, a first datum surface is disposed at a top surface of theadhesive profile defined by the raised peaks. In an example embodiment,the raised peaks are truncated to a top surface of the plurality ofoptical fibers. In some example embodiments, the raised peaks aretruncated to define a plurality of planar datum surfaces. In an exampleembodiment the fiber optic assembly also includes a lid disposed on thefirst datum surface. In some example embodiments, the lid includes atrench configured to receive at least a portion of the adhesive profileor the plurality of optical fibers. In an example embodiment the fiberoptic assembly also includes a photonic integrated circuit (PIC)substrate comprising a plurality of V-grooves configured to receive theraised peaks. In some example embodiments, the height of the adhesivebetween adjacent optical fibers is less than half of the diameter of anoptical fiber of the plurality of optical fibers

In a further embodiment, a fiber optic assembly is provided including asupport substrate having a planar surface and a signal-fiber arraysupported on the planer surface of the support substrate. Thesignal-fiber array includes a plurality of optical fibers. The fiberoptic assembly also includes an adhesive disposed on the plurality ofoptical fibers and the support substrate. Each of the optical fibers isspaced from adjacent optical fibers of the plurality of optical fibersand a datum surface is defined on an outer surface of each edge opticalfiber of the plurality of optical fibers.

In still a further example embodiment, a method of fabricating a fiberoptic assembly is provided including placing a signal-fiber array on afirst surface of a support substrate, the signal-fiber array including aplurality of optical fibers disposed in a plurality of V-groovesdisposed on the first surface of the support substrate, applying anadhesive on the plurality of optical fibers and the support substrate,pressing a release pad onto the adhesive, signal-fiber array, andsupport substrate, curing the adhesive, and removing the release pad. Afirst datum surface is disposed at a top of each of the plurality ofoptical fibers opposite the support surface.

In yet another example embodiment, a method of fabricating fiber opticassembly is provided including placing a signal-fiber array on a planarsurface of a support substrate, the signal-fiber array including aplurality of optical fibers, applying an adhesive on the plurality ofoptical fibers and the support substrate, pressing a release pad ontothe adhesive, signal-fiber array, and support substrate, curing theadhesive, and removing the release pad. Each optical fiber is spacedapart from adjacent optical fibers of the plurality of optical fibers ata precise pitch.

In an example embodiment the fiber optic assembly also includes aphotonic integrated circuit (PIC) substrate including a plurality ofwaveguides, and the plurality of optical fibers is aligned with theplurality of waveguides. In some example embodiments, the PIC substratecomprises a notch configured to receive the plurality of optical fiberstherein, and the datum surfaces on the edge optical fibers define aninterface of the plurality of optical fibers and the notch enablingpassive alignment of the plurality of optical fibers to the plurality ofwaveguides. In an example embodiment, the adhesive defines a spacerregion on the planar surface and the height of the spacer regioncorresponds to the depth of the waveguides below a top surface of thePIC substrate. In some example embodiments, the adhesive defines atleast one alignment feature configured to enable passive alignment ofthe fiber optic assembly. In an example embodiment, the alignmentfeature include a raised feature In some example embodiments, thealignment feature includes a recessed feature. In an example embodiment,the alignment feature is disposed at a precision horizontal and verticaloffset from the plurality of optical fibers.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It will be apparent to those skilled in the art that variousmodifications and variations may be made without departing from thespirit or scope of the invention. Since modifications combinations,sub-combinations and variations of the disclosed embodimentsincorporating the spirit and substance of the invention may occur topersons skilled in the art, the invention should be construed to includeeverything within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A method for fabricating a multifiber assembly,comprising: selecting a plurality of optical fibers that each have arespective cladding diameter; determining a maximum fiber core positionerror for the plurality of optical fibers in a plurality ofconfigurations, wherein for each configuration: the plurality of opticalfibers are arranged side-by-side in an array such that each opticalfiber has a position in the array, the plurality of optical fibers arearranged in a different order, each optical fiber has a respective fibercore position relative to an ideal core position for that optical fiberto define a respective fiber core position error, the ideal corepositions are based on each optical fiber of the plurality of opticalfibers having an ideal cladding diameter, and the maximum fiber coreposition error is a maximum of the respective fiber core positionerrors; and determining a desired order of the plurality of opticalfibers that minimizes the maximum fiber core position error.
 2. Themethod of claim 1, wherein for each optical fiber of the plurality ofoptical fibers, a geometric center of the respective cladding diameteris used as the respective fiber core position.
 3. The method of claim 1,further comprising: measuring the respective cladding diameter of eachoptical fiber of the plurality of optical fibers at one or morelocations along a length of the optical fiber.
 4. The method of claim 1,further comprising: arranging the plurality of optical fibers in thedesired order; and applying a matrix material to the plurality ofoptical fibers to form an optical fiber ribbon.
 5. The method of claim1, wherein the determining the maximum fiber core position errorcomprises: arranging the plurality of optical fibers in a firstconfiguration; determining a first maximum fiber core position error forthe first configuration; swapping the positions of two optical fibers ofthe plurality of optical fibers, such that the plurality of opticalfibers are in a second configuration; and determining a second maximumfiber core position error for the second configuration.
 6. The method ofclaim 5, wherein the swapping positions of two optical fibers of theplurality of optical fibers and the determining the maximum fiber coreposition error for a given configuration of the plurality ofconfigurations is repeated for a predetermined number of iterations. 7.The method of claim 6, wherein the predetermined number of iterationscomprises all possible configurations of the plurality of opticalfibers.
 8. The method of claim 6, wherein the predetermined number ofiterations comprises at least 100 iterations.
 9. The method of claim 1,further comprising: selecting a plurality of additional optical fibers,wherein the determining the maximum fiber core position error comprises:arranging the plurality of optical fibers in a first configuration;determining a first maximum fiber core position error for the firstconfiguration; swapping positions of two optical fibers of the pluralityof optical fibers and swapping a first optical fiber of the plurality ofoptical fibers with a second optical fiber of the plurality ofadditional optical fibers to form a second configuration; anddetermining a second maximum fiber core position error for the secondconfiguration.
 10. The method of claim 9, wherein each optical fiber ofthe plurality of optical fibers is colored and each of the additionaloptical fibers is colored, and wherein the swapping the first opticalfiber of the plurality of optical fibers with the second optical fiberof the plurality of additional optical fibers comprises swapping thefirst optical fiber with a second optical fiber of the same color. 11.The method of claim 9, wherein the swapping positions of two opticalfibers and the determining of a maximum fiber core position error isrepeated for a predetermined number of iterations.
 12. The method ofclaim 10, wherein the predetermined number of iterations comprises allpossible configurations of the plurality of optical fibers and theplurality of additional optical fibers.
 13. The method of claim 10,wherein the predetermined number of iterations comprises at least 100iterations.
 14. A method for fabricating a fiber array unit comprising:selecting a plurality of optical fibers that each have a respectivecladding diameter; determining a maximum fiber core position error forthe plurality of optical fibers in a plurality of configurations,wherein for each configuration: the plurality of optical fibers arearranged side-by-side in an array such that each optical fiber has aposition in the array, the plurality of optical fibers are arranged in adifferent order, each optical fiber has a respective fiber core positionrelative to an ideal core position for that optical fiber to define arespective fiber core position error, the ideal core positions are basedon each optical fiber of the plurality of optical fibers having an idealcladding diameter, and the maximum fiber core position error is amaximum of the respective fiber core position errors; and determining adesired order of the plurality of optical fibers that minimizes themaximum fiber core position error positioning the plurality of opticalfibers on a support substrate in the configuration that comprises thedesired order; and applying an adhesive to affix the plurality ofoptical fibers to the support substrate.
 15. The method of claim 14,wherein for each optical fiber of the plurality of optical fibers, ageometric center of the respective cladding diameter is used as therespective fiber core position.
 16. The method of claim 14, furthercomprising: measuring the respective cladding diameter of each opticalfiber of the plurality of optical fibers at one or more locations alonga length of the optical fiber.
 17. The method of claim 14, furthercomprising: arranging the plurality of optical fibers in the desiredorder; applying a matrix material to the plurality of optical fibers toform an optical fiber ribbon; removing the matrix material from aportion of the plurality of optical fibers positioned on the supportsubstrate.
 18. The method of claim 14, wherein the determining themaximum fiber core position error comprises: arranging the plurality ofoptical fibers in a first configuration; determining a first maximumfiber core position error for the first configuration; swapping thepositions of two optical fibers of the plurality of optical fibers, suchthat the plurality of optical fibers are in a second configuration; anddetermining a second maximum fiber core position error for the secondconfiguration.
 19. The method of claim 18, wherein the swappingpositions of two optical fibers of the plurality of optical fibers andthe determining of the maximum fiber core position error for a givenconfiguration of the plurality of configurations is repeated for apredetermined number of iterations.
 20. The method of claim 14, furthercomprising: selecting a plurality of additional optical fibers, whereinthe determining the maximum fiber core total error comprises: arrangingthe plurality of optical fibers in a first configuration; determining afirst maximum fiber core position error for the first configuration;swapping positions of two optical fibers of the plurality of opticalfibers or swapping a first optical fiber of the plurality of opticalfibers with a second optical fiber of the plurality of additionaloptical fibers to form a second configuration; and determining a secondmaximum fiber core position error for the second configuration.
 21. Themethod of claim 20, wherein the swapping positions of two optical fibersand the determining of a current maximum fiber core total error for acurrent configuration is repeated for a predetermined number ofiterations.
 22. The method of claim 14, wherein the plurality of opticalfibers comprises a first plurality of optical fibers and the desiredorder comprises a first desired order, the method further comprises:selecting a second plurality of optical fibers that each have arespective cladding diameter; determining a plurality of maximum fibercore position errors for the second plurality of optical fibers in asecond plurality of configurations; determining a second determinedorder of the second plurality of optical fibers that minimizes themaximum fiber core total error; positioning the second plurality ofoptical fibers in the second determined order; interdigitating the firstplurality of optical fibers in the first desired order and the secondplurality of optical fibers in the second desired order to form ainterdigitated group of optical fibers; positioning the interdigitatedgroup of optical fibers on the support substrate; and applying theadhesive to affix the interdigitated group of optical fibers to thesupport substrate.
 23. The method of claim 22, wherein theinterdigitated group of optical fibers comprises a first interdigitatedgroup of optical fibers and the method further comprises: positioning asecond interdigitated group of optical fibers on the support substrate;and applying the adhesive to affix the second interdigitated group ofoptical fibers to the support substrate.
 24. The method of claim 23,wherein the second interdigitated group of optical fibers is in directcontact with the first interdigitated group of optical fibers.
 25. Themethod of claim 23, wherein the second interdigitated group of opticalfibers is spaced apart from the first interdigitated group of opticalfibers and the method further comprises: positioning a spacer fiber onthe support substrate between the first interdigitated group of opticalfibers and the second interdigitated group of optical fibers; andapplying the adhesive to affix the spacer fiber to the supportsubstrate.